[Core] get_keycode_string(): function to format keycodes as strings, for more readable debug logging.

[getreuer]

This PR adds get_keycode_string() as described in https://getreuer.info/posts/keyboards/keycode-string/index.html as a utility function in core.

Description

It's much nicer to read keycodes as names like "LT(2,KC_D)" than numerical codes like "0x4207." This PR adds a function get_keycode_string() for that purpose, based on my userspace implementation linked above.

How to use it

Add in rules.mk:

KEYCODE_STRING_ENABLE = yes

Then use the get_keycode_string(kc) function to convert a given 16-bit keycode to a human-readable string. Example:

uint16_t keycode = C(KC_PGUP);
const char *key_name = get_keycode_string(keycode);
dprintf("kc: %s\n", key_name); // Logs "kc: C(KC_PGUP)"

Limitations

• Many common QMK keycodes are recognized by get_keycode_string(), but not all. These include some common basic keycodes, layer switch keycodes, mod-taps, one-shot keycodes, tap dance keycodes, and Unicode keycodes. As a fallback, an unrecognized keycode is written as a hex number. Names for additional keycodes can also be added, see below.

• Keycodes are stringified according to the default KC_-prefixed keycodes, which is suboptimal when the computer is configured to a non-US-QWERTY layout.

Defining names for additional keycodes

Optionally, keycode_string_names_user or keycode_string_names_kb may be defined to add names for additional keycodes. For example, supposing keymap.c defines MYMACRO1 and MYMACRO2 as custom keycodes, the following adds their names:

const keycode_string_name_t keycode_string_names_user[] = {
    KEYCODE_STRING_NAME(MYMACRO1),
    KEYCODE_STRING_NAME(MYMACRO2),
    KEYCODE_STRING_NAMES_END // End of table sentinel.
};

Types of Changes

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

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).

[getreuer]

Thanks for the quick review, @drashna! I've updated the PR according to your comments.