add weak attribute to send_char

[NapOli1084]

Description

Adds __attribute__((weak)) to send_char().
Allows users to override it in order to use a custom delay based on some state of the keyboard.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• Resolves [Feature Request] Need a way to configure unicode tap code delay #26304

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[drashna]

Given the issue that this is trying to solve, a better solution would be to use send_char_with_delay in the unicode functions, rather than making send_char weak.

This way, you can use a different/custom delay with the unicode code, rather than making issues with a bunch of other code paths that depend on send_char

[NapOli1084]

That's pretty much what my first idea was. I'll mark this PR as draft for now and attempt one with such a delay. If we can come up with a solution you're fine with, I'll drop this one.