Add command to get evaluated color setting (#14683)

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# Description
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In #14647 I added the option `"auto"` to be a valid option for
`$env.config.use_ansi_coloring`. That improves the decision making
whether ansi colors should be used or not but that makes it hard for
custom commands to respect that value as the config might now be a
non-boolean value. To retrieve that evaluated value I added a new
command called `config use-colors` that returns an evaluated boolean
that may be used to decide if colors should be used or not.

# User-Facing Changes
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Scripts that previously just checked `$env.config.use_ansi_coloring`
should now use `config use-colors` for their color decision making.

# Tests + Formatting
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This PR essentially only runs `UseAnsiColoring::get`, and that is highly
tested in the #14647, so I don't think this needs further testing.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
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I'm not sure if we have any docs about that ansi coloring setup. If we
have, we should update these.

crates/nu-command/src/default_context.rs

@@ -354,6 +354,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             ConfigFlatten,
             ConfigMeta,
             ConfigReset,
+            ConfigUseColors,
         };
 
         // Math

crates/nu-command/src/env/config/config_use_colors.rs

@@ -0,0 +1,41 @@
+use nu_engine::command_prelude::*;
+
+#[derive(Clone)]
+pub struct ConfigUseColors;
+
+impl Command for ConfigUseColors {
+    fn name(&self) -> &str {
+        "config use-colors"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .category(Category::Env)
+            .input_output_type(Type::Nothing, Type::Bool)
+    }
+
+    fn description(&self) -> &str {
+        "Get the configuration for color output."
+    }
+
+    fn extra_description(&self) -> &str {
+        r#"Use this command instead of checking `$env.config.use_ansi_coloring` to properly handle the "auto" setting, including environment variables that influence its behavior."#
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let use_ansi_coloring = engine_state
+            .get_config()
+            .use_ansi_coloring
+            .get(engine_state);
+        Ok(PipelineData::Value(
+            Value::bool(use_ansi_coloring, call.head),
+            None,
+        ))
+    }
+}

crates/nu-command/src/env/config/mod.rs

@@ -3,9 +3,11 @@ mod config_env;
 mod config_flatten;
 mod config_nu;
 mod config_reset;
+mod config_use_colors;
 
 pub use config_::ConfigMeta;
 pub use config_env::ConfigEnv;
 pub use config_flatten::ConfigFlatten;
 pub use config_nu::ConfigNu;
 pub use config_reset::ConfigReset;
+pub use config_use_colors::ConfigUseColors;

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

@@ -9,6 +9,7 @@ pub use config::ConfigFlatten;
 pub use config::ConfigMeta;
 pub use config::ConfigNu;
 pub use config::ConfigReset;
+pub use config::ConfigUseColors;
 pub use export_env::ExportEnv;
 pub use load_env::LoadEnv;
 pub use source_env::SourceEnv;