Add specific parse errors for angle-bracket generics

[AaronC81]

This PR adds specific parse errors for these cases, where a developer new to Gleam (i.e. me!) tries to write generic functions or types using angle brackets:

fn something<T>(x: T) { ... }

let x: List<String> = []

type Maybe<String> { ... }

See the snapshots for examples of the concrete errors.

First PR, so any stylistic comments are welcome :)
Thanks for the great contributing documentation, it was very easy to get started.

Closes #4816.

[AaronC81]

Should I add a CHANGELOG entry for this change?
I can see some PRs add one, but not all, and I couldn't spot any docs on which kinds of changes need an entry.

If there should be one, let me know and I'm happy to update the PR

[GearsDatapacks]

Yes, you'll need to add a changelog entry. Since we're in the release candidate phase for 1.12, this won't be merged until after 1.12 is out.
You'll need to add a new "Unreleased" section at the top of the file (h2) and then add a "Compiler" section below that (h3). Then you can look at existing changelog entries for an idea of what to write

[AaronC81]

@GearsDatapacks - Great, thanks - added