Disallow ^ in def command names (#7606)

# Description

Closes #7273.

Also slightly edits/tidies up parser.rs.

# User-Facing Changes

`^` is now forbidden in `def` and `def-env` command names. EDIT: also
`alias`.

# 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-parser/src/parse_keywords.rs

@@ -58,6 +58,7 @@ pub fn parse_def_predecl(
         working_set.exit_scope();
         if let (Some(name), Some(mut signature)) = (name, signature) {
             if name.contains('#')
+                || name.contains('^')
                 || name.parse::<bytesize::ByteSize>().is_ok()
                 || name.parse::<f64>().is_ok()
             {
@@ -661,6 +662,7 @@ pub fn parse_alias(
 
                 let checked_name = String::from_utf8_lossy(&alias_name);
                 if checked_name.contains('#')
+                    || checked_name.contains('^')
                     || checked_name.parse::<bytesize::ByteSize>().is_ok()
                     || checked_name.parse::<f64>().is_ok()
                 {