feat: allow unconstrained after visibility

[asterite]

Description

Problem

We currently parse unconstrained pub fn ... but ideally we'd want it to be pub unconstrained fn ...

See this comment too: #6180 (comment)

Summary

This PR implements the above, but actually all of these:

• We now allow unconstrained pub fn ... and pub unconstrained fn (the former for backwards compatibility)
• The formatter will change unconstrained pub fn to pub unconstrained fn (so after some time we can remove the backwards-compatibility case)
• The formatter will now format a function "header" (I didn't know what to call it): that's the fn outer comments, attributes, and modifiers. For example before this PR if you wrote pub fn foo() {} it was left untouched, but now it's changed to pub fn foo() {}.

Additional Context

None.

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.