[Feature Request] Add mod key shortcuts and "_T" functions for multiple modifier combinations

[nopunin10did]

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

QMK (and by extension, other apps based on QMK like QMK Configurator and Oryx) currently make it a bit inconvenient to add certain modifier combinations without using the ANY() function. Likewise, the following combinations do not exist in the list of current "_T" functions to add differing hold/tap behavior to keys.

1. LSA(kc) & LSA_T(kc): Left Shift + Left Alt is used by text editors for vertical text selection.
2. RSA(kc) & RSA_T(kc): Right Shift + Right Alt is used by several locales where Shift + Alt Gr produces specific character outputs.
3. RCS(kc) & RCS_T(kc): Right Ctrl + Right Shift is used more rarely, but in some locales the right-side control modifier is treated like a Kana key and accesses otherwise unavailable characters. The Canadian Multilingual keyboard layout is probably the best example.

[nopunin10did]

I'm currently coding this and should have a PR soon.