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.

src/tests/test_engine.rs

@@ -319,6 +319,11 @@ fn default_value12() -> TestResult {
     fail_test(r#"def foo [--x:int = "a"] { $x }"#, "default value not int")
 }
 
+#[test]
+fn default_value_expression() -> TestResult {
+    run_test(r#"def foo [x = ("foo" | str length)] { $x }; foo"#, "3")
+}
+
 #[test]
 fn loose_each() -> TestResult {
     run_test(r#"[[1, 2, 3], [4, 5, 6]] | each { $in.1 } | math sum"#, "7")