fixes a def parsing bug with a default list (#8096)

# Description

This PR fixes a bug where a default list in a custom command parameter
wasn't being accepted. The reason was because it was comparing specific
types of list like `list<any>` != `list<string>`. So, this PR attempts
to fix that.

### Before

```
> def f [param: list = [one]] { echo $param }
Error: nu::parser::assignment_mismatch (link)

  × Default value wrong type
   ╭─[entry #1:1:1]
 1 │ def f [param: list = [one]] { echo $param }
   ·                      ──┬──
   ·                        ╰── default value not list<any>
   ╰────
```

### After

```
> def f [param: list = [one]] {echo $param}
> f
╭───┬─────╮
│ 0 │ one │
╰───┴─────╯
```

closes #8092 

# User-Facing Changes



# 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-protocol/src/ty.rs

@@ -61,6 +61,10 @@ impl Type {
         matches!(self, Type::Int | Type::Float | Type::Number)
     }
 
+    pub fn is_list(&self) -> bool {
+        matches!(self, Type::List(_))
+    }
+
     /// Does this type represent a data structure containing values that can be addressed using 'cell paths'?
     pub fn accepts_cell_paths(&self) -> bool {
         matches!(self, Type::List(_) | Type::Record(_) | Type::Table(_))