Added Emacs as a unicode input mode

[chewxy]

I added a new unicode input mode: emacs. (see also: PR in the main qmk branch: qmk#16949)

While strictly not an operating system, inserting unicode characters in emacs has its own keybinding. The most usual way is C-x-8 RET followed by the unicode in hex. As a person who works across different OSes but with the constant being emacs, this was the most straightforward way forwards.

Of course, the main credit goes to TrentinQuarantino in this emacs stack exchange post.

Description

I added a new unicode input mode in quantum/process_unicode_common.(c|h). Then I updated all the programmer friendly numbers of the enums, and added documentation too.

Types of Changes

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

Issues Fixed or Closed by This PR

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).

Additional notes

I'm unfamiliar with the testing protocols of this project. If I could get some help that'd be great. I'm reasonably familiar with C. However, what I have done is I have flashed two ergodox-EZs with this and tested it on emacs on both linux and macos. My Windows computer isn't quite available right now, so I'm unable to test on Windows.

[drashna]

Generally, we dont accept PRs directly. They should go through QMK first, and then will get pulled down from there.