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-protocol/src/parse_error.rs

@@ -38,13 +38,6 @@ pub enum ParseError {
     #[diagnostic(code(nu::parser::parse_mismatch))]
     Expected(String, #[label("expected {0}")] Span),
 
-    #[error("Missing || inside closure")]
-    #[diagnostic(
-        code(nu::parser::closure_missing_pipe),
-        help("Try add || to the beginning of closure")
-    )]
-    ClosureMissingPipe(#[label("Parsing as a closure, but || is missing")] Span),
-
     #[error("Type mismatch during operation.")]
     #[diagnostic(code(nu::parser::type_mismatch))]
     Mismatch(String, String, #[label("expected {0}, found {1}")] Span), // expected, found, span
@@ -510,7 +503,6 @@ impl ParseError {
             ParseError::UnknownOperator(_, _, s) => *s,
             ParseError::InvalidLiteral(_, _, s) => *s,
             ParseError::NotAConstant(s) => *s,
-            ParseError::ClosureMissingPipe(s) => *s,
         }
     }
 }