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-command/tests/commands/alias.rs

@@ -43,35 +43,38 @@ fn alias_hiding_2() {
 
 #[test]
 fn alias_fails_with_invalid_name() {
+    let err_msg = "alias name can't be a number, a filesize, or contain a hash # or caret ^";
     let actual = nu!(
-        cwd: "tests/fixtures/formats", pipeline(
+        cwd: ".", pipeline(
         r#"
             alias 1234 = echo "test"   
         "#
     ));
-    assert!(actual
-        .err
-        .contains("alias name can't be a number, a filesize, or contain a hash"));
+    assert!(actual.err.contains(err_msg));
 
     let actual = nu!(
-        cwd: "tests/fixtures/formats", pipeline(
+        cwd: ".", pipeline(
         r#"
             alias 5gib = echo "test"   
         "#
     ));
-    assert!(actual
-        .err
-        .contains("alias name can't be a number, a filesize, or contain a hash"));
+    assert!(actual.err.contains(err_msg));
 
     let actual = nu!(
-        cwd: "tests/fixtures/formats", pipeline(
+        cwd: ".", pipeline(
         r#"
             alias "te#t" = echo "test"   
         "#
     ));
-    assert!(actual
-        .err
-        .contains("alias name can't be a number, a filesize, or contain a hash"));
+    assert!(actual.err.contains(err_msg));
+
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            alias ^foo = "bar"
+        "#
+    ));
+    assert!(actual.err.contains(err_msg));
 }
 
 #[test]