Add themes to help command when available #10318 (#10623)

# Description
The issue #10318 is resolved by introducing helper methods within the
existing `get_documentation` function in the nu-engine crate. Initially,
I considered using nu-color-config crate to convert HEX config color to
ANSI color and employing the following method
[https://github.com/nushell/nushell/blob/main/crates/nu-color-config/src/color_config.rs#L9C1-L20C2](https://github.com/nushell/nushell/blob/main/crates/nu-color-config/src/color_config.rs#L9C1-L20C2).
However, this approach was deemed impractical due to circular
dependencies. Consequently, in a manner akin to how we invoke the
`table` command from the nu-command crate in `get_documentation`
function to create a themed-colored table, we invoke the `ansi` command
from nu-command to obtain the ANSI theme color code.

# User-Facing Changes
Visual Changes Only: the help command now uses configured theme, else it
falls back on default hard coded values.


# Tests + Formatting
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crates/nu-plugin/src/plugin/mod.rs

@@ -385,7 +385,7 @@ fn print_help(plugin: &mut impl Plugin, encoder: impl PluginEncoder) {
                 }
             })
             .and_then(|_| {
-                let flags = get_flags_section(&signature.sig, |v| format!("{:#?}", v));
+                let flags = get_flags_section(None, &signature.sig, |v| format!("{:#?}", v));
                 write!(help, "{flags}")
             })
             .and_then(|_| writeln!(help, "\nParameters:"))