Relax the closure syntax, highlight differently (#8846)

# Description

This relaxes the closure syntax so that `||` is no longer required. This
allows for `ls | each { $in.name }` for example.

I've gone ahead and changed the syntax highlighting so that blocks and
closures are distinct for now.

# User-Facing Changes

Removes `||` requirement for closures.

# 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
- `cargo run -- crates/nu-std/tests/run.nu` to run the tests for the
standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# 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-utils/src/sample_config/default_config.nu

@@ -57,6 +57,7 @@ let dark_theme = {
     shape_binary: purple_bold
     shape_block: blue_bold
     shape_bool: light_cyan
+    shape_closure: green_bold
     shape_custom: green
     shape_datetime: cyan_bold
     shape_directory: cyan
@@ -140,6 +141,7 @@ let light_theme = {
     shape_binary: purple_bold
     shape_block: blue_bold
     shape_bool: light_cyan
+    shape_closure: green_bold
     shape_custom: green
     shape_datetime: cyan_bold
     shape_directory: cyan