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-parser/src/parser.rs

@@ -1589,10 +1589,8 @@ pub fn parse_brace_expr(
 
     if matches!(second_token, None) {
         // If we're empty, that means an empty record or closure
-        if matches!(shape, SyntaxShape::Closure(None)) {
-            parse_closure_expression(working_set, shape, span, false)
-        } else if matches!(shape, SyntaxShape::Closure(Some(_))) {
-            parse_closure_expression(working_set, shape, span, true)
+        if matches!(shape, SyntaxShape::Closure(_)) {
+            parse_closure_expression(working_set, shape, span)
         } else if matches!(shape, SyntaxShape::Block) {
             parse_block_expression(working_set, span)
         } else if matches!(shape, SyntaxShape::MatchBlock) {
@@ -1603,13 +1601,11 @@ pub fn parse_brace_expr(
     } else if matches!(second_token_contents, Some(TokenContents::Pipe))
         || matches!(second_token_contents, Some(TokenContents::PipePipe))
     {
-        parse_closure_expression(working_set, shape, span, true)
+        parse_closure_expression(working_set, shape, span)
     } else if matches!(third_token, Some(b":")) {
         parse_full_cell_path(working_set, None, span)
-    } else if matches!(shape, SyntaxShape::Closure(None)) {
-        parse_closure_expression(working_set, shape, span, false)
-    } else if matches!(shape, SyntaxShape::Closure(Some(_))) || matches!(shape, SyntaxShape::Any) {
-        parse_closure_expression(working_set, shape, span, true)
+    } else if matches!(shape, SyntaxShape::Closure(_)) || matches!(shape, SyntaxShape::Any) {
+        parse_closure_expression(working_set, shape, span)
     } else if matches!(shape, SyntaxShape::Block) {
         parse_block_expression(working_set, span)
     } else if matches!(shape, SyntaxShape::MatchBlock) {
@@ -4201,7 +4197,6 @@ pub fn parse_closure_expression(
     working_set: &mut StateWorkingSet,
     shape: &SyntaxShape,
     span: Span,
-    require_pipe: bool,
 ) -> Expression {
     trace!("parsing: closure expression");
 
@@ -4275,15 +4270,7 @@ pub fn parse_closure_expression(
             Some((Box::new(Signature::new("closure".to_string())), *span)),
             1,
         ),
-        _ => {
-            if require_pipe {
-                working_set.error(ParseError::ClosureMissingPipe(span));
-                working_set.exit_scope();
-                return garbage(span);
-            } else {
-                (None, 0)
-            }
-        }
+        _ => (None, 0),
     };
 
     // TODO: Finish this