LSP: generate doc comments for trait functions

[sdankel]

Description

Closes #4645

This PR lets the user generate doc comments for ABI methods. Previously it was working for functions and function implementations of ABI/trait, but not on the ABI/trait definition itself.

Since the output is exactly the same for FunctionDecl to TraitFn, I wanted to do this in a way where they could share the code. I ended up writing a trait called FunctionSignature that both types implement that exposes parameters() and return_type(). This allows me to use the same code for code actions and I suspect it will come in handy in the future.

One problem was that FunctionDecl's return_type was TypeArgument while TraitFn had its return type as TypeId with an additional return_type_span field. I changed it so that TraitFn's return_type is now TypeArgument and removed return_type_span.

I also needed to add a span field to TraitFn since previously its span() was returning only the span of the function name, not the whole function. From what I could see, this doesn't impact the spans of any existing errors.

Testing

I added an integration test to the LSP for this scenario.

Example

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[Braqzen]

I'm just here for the vibes and support, go team!