tighter restrictions on alias and def names (#7392)

# Description

Prevent a situation where a `def` can't be run due to a poor choice of
name. Related: #6335. Hashtags, numbers and filesizes are no longer
allowed. `alias` check has been moved because previously `alias 123`
would be caught but `alias "123"` would be permitted.

# User-Facing Changes

Some definitions can no longer be made, but because they couldn't be run
previously anyway, it doesn't really matter.

# 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

@@ -51,7 +51,8 @@ fn alias_fails_with_invalid_name() {
     ));
     assert!(actual
         .err
-        .contains("alias name can't be a number or a filesize"));
+        .contains("alias name can't be a number, a filesize, or contain a hash"));
+
     let actual = nu!(
         cwd: "tests/fixtures/formats", pipeline(
         r#"
@@ -60,7 +61,17 @@ fn alias_fails_with_invalid_name() {
     ));
     assert!(actual
         .err
-        .contains("alias name can't be a number or a filesize"));
+        .contains("alias name can't be a number, a filesize, or contain a hash"));
+
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            alias "te#t" = echo "test"   
+        "#
+    ));
+    assert!(actual
+        .err
+        .contains("alias name can't be a number, a filesize, or contain a hash"));
 }
 
 #[test]