Improve alias highlighting/completions (#3594)

* Improve alias highlighting/completions

* Add example

crates/nu-command/src/commands/alias.rs

@@ -0,0 +1,40 @@
+use crate::prelude::*;
+use nu_engine::WholeStreamCommand;
+
+use nu_errors::ShellError;
+use nu_protocol::{Signature, SyntaxShape};
+
+pub struct Alias;
+
+impl WholeStreamCommand for Alias {
+    fn name(&self) -> &str {
+        "alias"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("alias")
+            .required("name", SyntaxShape::String, "the name of the alias")
+            .required("equals", SyntaxShape::String, "the equals sign")
+            .rest(SyntaxShape::Any, "the expansion for the alias")
+    }
+
+    fn usage(&self) -> &str {
+        "Alias a command to an expansion."
+    }
+
+    fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
+        alias(args)
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Alias ll to ls -l",
+            example: "alias ll = ls -l",
+            result: None,
+        }]
+    }
+}
+
+pub fn alias(_: CommandArgs) -> Result<OutputStream, ShellError> {
+    Ok(OutputStream::empty())
+}

crates/nu-command/src/commands/default_context.rs

@@ -16,6 +16,7 @@ pub fn create_default_context(interactive: bool) -> Result<EvaluationContext, Bo
             whole_stream_command(LoadEnv),
             whole_stream_command(Def),
             whole_stream_command(Source),
+            whole_stream_command(Alias),
             // System/file operations
             whole_stream_command(Exec),
             whole_stream_command(Pwd),