Enforce rust keyword order

[P-E-P]

Rust keyword should be used in a given order

1. pub
2. default
3. const
4. async
5. unsafe
6. extern

Rust provides multiple messages and notes to help with that:

5 | extern const pub fn myfun() {
  | -------^^^^^
  | |      |
  | |      expected `fn`
  | help: `const` must come before `extern`: `const extern`
  |
  = note: keyword order for functions declaration is `pub`, `default`, `const`, `async`, `unsafe`, `extern`

• We do have some FunctionQualifier structure to store the state of a function but not all those informations are kept in it.
• We parse in different ways those qualifiers and provide multiple error message.

We should probably unify all those under one same structure (FunctionQualifier) and create a function to parse a FunctionQualifier that could be used everytime we expect a function.

This function could emit an error as well as a note (rust_note) when the syntax is wrong.

Choose an appropriate API for this function. Should it return an Optional<FunctionQualifier> ? Always return a FunctionQualifier with private/safe/sync attributes ? Probably the latter.

[P-E-P]

Maybe we should wait for #2815 to be merged ? Also, rustc use unified Function and ExternalFunction, maybe we should do the same since function body is now optional ?

[braw-lee]

I will try this