feat(macros): Add TopLevelItem and Type types for quoting top-level definitions and types

[jfecher]

Description

Problem*

Resolves

Summary*

Adds a TopLevelItem and Type keywords & types for quoting type definitions, functions, trait impls, etc. and types respectively. The former is needed to identify items which can be unquoted at the top-level of a file - e.g. to prevent random expressions from being unquoted at the top level and to prevent trait impls from being unquoted in the middle of a function.

Additional Context

There is no way currently to create a TopLevelItem. The intended method is to be able to specify the type you want to quote in a quote expression: quote TopLevelItem { ... } but our current parser makes this very difficult since the top level statement parser would need to become recursive and be passed down through each function. For now I'm planning to have compiler API function's as a work around - e.g:

impl TopLevelItem {
    #[builtin(new_trait_impl)]
    pub fn new_trait_impl(generics: Vec<String>, trait: Type, typ: Type, methods: Vec<TopLevelItem>) -> TopLevelItem { }
}

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.