Add 'help escapes' command for quick reference of nushell string escapes (#10522)

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resolves #4869 

# Description
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Adds a `help escape` command that can be used to display a table of
string escape sequences and their outputs.
```nu
help escapes
```
```nu
help escapes -h
```

The command should also appear in the list displayed when tab
autocompleting on `help`.

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
Users can now use a new `help escapes` command to output a table of
string escape sequences and their outputs.

# Tests + Formatting
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# After Submitting
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Need to update docs to reflect existence of the new `help escapes`
command.

crates/nu-command/src/default_context.rs

@@ -1,7 +1,7 @@
 use nu_protocol::engine::{EngineState, StateWorkingSet};
 
 use crate::{
-    help::{HelpAliases, HelpCommands, HelpExterns, HelpModules, HelpOperators},
+    help::{HelpAliases, HelpCommands, HelpEscapes, HelpExterns, HelpModules, HelpOperators},
     *,
 };
 pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
@@ -127,6 +127,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             HelpCommands,
             HelpModules,
             HelpOperators,
+            HelpEscapes,
         };
 
         // Debug

crates/nu-command/src/help/help_escapes.rs

@@ -0,0 +1,156 @@
+use nu_protocol::{
+    ast::Call,
+    engine::{Command, EngineState, Stack},
+    record, Category, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Type,
+    Value,
+};
+
+#[derive(Clone)]
+pub struct HelpEscapes;
+
+impl Command for HelpEscapes {
+    fn name(&self) -> &str {
+        "help escapes"
+    }
+
+    fn usage(&self) -> &str {
+        "Show help on nushell string escapes."
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("help escapes")
+            .category(Category::Core)
+            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .allow_variants_without_examples(true)
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let head = call.head;
+        let escape_info = generate_escape_info();
+        let mut recs = vec![];
+
+        for escape in escape_info {
+            recs.push(Value::record(
+                record! {
+                    "sequence" => Value::string(escape.sequence, head),
+                    "output" => Value::string(escape.output, head),
+                },
+                head,
+            ));
+        }
+
+        Ok(recs
+            .into_iter()
+            .into_pipeline_data(engine_state.ctrlc.clone()))
+    }
+}
+
+struct EscapeInfo {
+    sequence: String,
+    output: String,
+}
+
+fn generate_escape_info() -> Vec<EscapeInfo> {
+    vec![
+        EscapeInfo {
+            sequence: "\\\"".into(),
+            output: "\"".into(),
+        },
+        EscapeInfo {
+            sequence: "\\\'".into(),
+            output: "\'".into(),
+        },
+        EscapeInfo {
+            sequence: "\\\\".into(),
+            output: "\\".into(),
+        },
+        EscapeInfo {
+            sequence: "\\/".into(),
+            output: "/".into(),
+        },
+        EscapeInfo {
+            sequence: "\\(".into(),
+            output: "(".into(),
+        },
+        EscapeInfo {
+            sequence: "\\)".into(),
+            output: ")".into(),
+        },
+        EscapeInfo {
+            sequence: "\\{".into(),
+            output: "{".into(),
+        },
+        EscapeInfo {
+            sequence: "\\}".into(),
+            output: "}".into(),
+        },
+        EscapeInfo {
+            sequence: "\\$".into(),
+            output: "$".into(),
+        },
+        EscapeInfo {
+            sequence: "\\^".into(),
+            output: "^".into(),
+        },
+        EscapeInfo {
+            sequence: "\\#".into(),
+            output: "#".into(),
+        },
+        EscapeInfo {
+            sequence: "\\|".into(),
+            output: "|".into(),
+        },
+        EscapeInfo {
+            sequence: "\\~".into(),
+            output: "~".into(),
+        },
+        EscapeInfo {
+            sequence: "\\a".into(),
+            output: "alert bell".into(),
+        },
+        EscapeInfo {
+            sequence: "\\b".into(),
+            output: "backspace".into(),
+        },
+        EscapeInfo {
+            sequence: "\\e".into(),
+            output: "escape".into(),
+        },
+        EscapeInfo {
+            sequence: "\\f".into(),
+            output: "form feed".into(),
+        },
+        EscapeInfo {
+            sequence: "\\n".into(),
+            output: "newline (line feed)".into(),
+        },
+        EscapeInfo {
+            sequence: "\\r".into(),
+            output: "carriage return".into(),
+        },
+        EscapeInfo {
+            sequence: "\\t".into(),
+            output: "tab".into(),
+        },
+        EscapeInfo {
+            sequence: "\\u{X...}".into(),
+            output: "a single unicode character, where X... is 1-6 digits (0-9, A-F)".into(),
+        },
+    ]
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn test_examples() {
+        use super::HelpEscapes;
+        use crate::test_examples;
+        test_examples(HelpEscapes {})
+    }
+}

crates/nu-command/src/help/mod.rs

@@ -1,6 +1,7 @@
 mod help_;
 mod help_aliases;
 mod help_commands;
+mod help_escapes;
 mod help_externs;
 mod help_modules;
 mod help_operators;
@@ -8,6 +9,7 @@ mod help_operators;
 pub use help_::Help;
 pub use help_aliases::HelpAliases;
 pub use help_commands::HelpCommands;
+pub use help_escapes::HelpEscapes;
 pub use help_externs::HelpExterns;
 pub use help_modules::HelpModules;
 pub use help_operators::HelpOperators;