core: better tests for parsers/jsx

[PatrickJS]

• We need a test to fail when we add new features or change the way the parser works that may break it
• We need some documentation on how it works and what patterns are preferred

A few quick videos would be a quick way to document this for current contributors and any future ones trying to jump in. When the codebase is more mature, or by demand, we can put more effort into documenting this more than just videos