New keycode macro (XP) for shifted character pairs using UNICODEMAP + bug fixes and...

New keycode macro (XP) for shifted character pairs using UNICODEMAP + bug fixes and improvements (#4803)

* Expose unicode_saved_mods

* Add UNICODEMAP shift pair functionality and XS keycode

* Add XS to keycode reference documentation

* Pick pair index based on both Shift and Caps Lock state

* Add XS to Unicode feature docs

* Clean up process_unicode* headers

* Extract unicode_map index calculation into function

* Pick pair index as XOR rather than OR of Shift and Caps states

* unicode_input_start() has to be called before the unicode_map index is calculated

* Replace unicodemap_input_error() with more generic unicode_input_cancel()

* Replace register+tap+unregister with tap_code16(LCTL(LSFT(KC_U)))

* UNICODE_OSX_KEY → UNICODE_KEY_OSX, UNICODE_WINC_KEY → UNICODE_KEY_WINC

* Make keycode range checks more robust

* Fix keycode range checks for different input modes

* Add UNICODE_KEY_LNX, update docs

* QK_UNICODEMAP_SHIFT → QK_UNICODEMAP_PAIR

* XS → XP, update docs

* Tweak Unicode docs

* Use recently added MOD_MASK_SHIFT and IS_HOST_LED_ON helpers

* Update Unicode table in docs/keycodes.md

* Update Unicode docs per review comments

* Replace references to Mac OS X with macOS in Unicode docs

* As of v0.9.0, WinCompose supports all possible code points

* Expand descriptions in XP docs

* Update keycode table and cycling docs

* Further expand cycling docs