Clean up keyword lines in help (#7243)

# Description

This makes the help messages cleaner for keyword-style arguments.

Before:
```
  (optional) else_expression <Keyword([101, 108, 115, 101], Expression)>: expression or block to run if check fails
```

Now:
```
  (optional) "else" + <expression>: expression or block to run if check fails
```


# User-Facing Changes

Changes how help is printed, so we use slightly different shape names

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-plugin/src/plugin/mod.rs

@@ -329,7 +329,7 @@ fn print_help(plugin: &mut impl Plugin, encoder: impl PluginEncoder) {
                     .try_for_each(|positional| {
                         writeln!(
                             help,
-                            "  {} <{:?}>: {}",
+                            "  {} <{}>: {}",
                             positional.name, positional.shape, positional.desc
                         )
                     })
@@ -341,7 +341,7 @@ fn print_help(plugin: &mut impl Plugin, encoder: impl PluginEncoder) {
                     .try_for_each(|positional| {
                         writeln!(
                             help,
-                            "  (optional) {} <{:?}>: {}",
+                            "  (optional) {} <{}>: {}",
                             positional.name, positional.shape, positional.desc
                         )
                     })
@@ -350,7 +350,7 @@ fn print_help(plugin: &mut impl Plugin, encoder: impl PluginEncoder) {
                 if let Some(rest_positional) = &signature.rest_positional {
                     writeln!(
                         help,
-                        "  ...{} <{:?}>: {}",
+                        "  ...{} <{}>: {}",
                         rest_positional.name, rest_positional.shape, rest_positional.desc
                     )
                 } else {