Support heapless::String

[mmalecki]

Goal

Type generation for heapless::String, unrelated to an existing issue.

Changes

• Add an impl_shadow for heapless::String
• Fix shadowing of String in macros
• Add a test

Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.

[gustavo-shigueo]

Hi @mmalecki, thanks for the PR! Wow, somehow it never even ocurred to me that the return types could also get shadowed, I pretty much only ever worried about method calls such as <T as ::ts_rs::TS>::inline() lol