[Feature Request] New extra-keycodes for Japanese JIS-kana-input

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Feature Request Type

• Core functionality
• Add-on hardware support (eg. audio, RGB, OLED screen, etc.)
• Alteration (enhancement/optimization) of existing feature(s)
• New behavior

Description

I would like to request the addition of extra keycode aliases for JIS-kana-input in the Japanese layout. Currently, QMK supports JIS keyboard layouts for rōmaji mode, but there is no built-in support for direct Kana input keycodes. This feature would benefit users who prefer Kana input over rōmaji input.

Kana input users need to assign keycodes corresponding to each kana character. Since kana input differs significantly from QWERTY-based rōmaji input, defining custom keycodes would allow for a more seamless experience when using QMK-powered keyboards.

I propose introducing custom keycodes for Kana input and defining them in quantum/keymap_extras/keymap_japanese_kana.h. The keycodes should have the JK_ prefix (short for "Japanese Kana") to distinguish them from other keycodes.

For example:

• JK_A = KC_3 (corresponding to あ = a)
• JK_KA = KC_T (corresponding to か = ka)
• JK_VOICED = JP_AT = KC_LBRC (corresponding to ゛ = voiced consonant mark)

Some references for Kana input mappings below:

JIS Kana layout: Wikipedia - Japanese input method

I am planning to implement this feature myself and submit a PR. If there are any suggestions or concerns regarding this issue, please let me know. Thank you!