Windows input: Fix wrong OEM key, add missing keys, cleanup

[mhilbrunner]

Superseded by #56514

This cleans up Windows key input mapping, fixes the braces/brackets mixup for the OEM keys, adds additional missing keys and mappings from WinUser.h and adds more docs/comments.

Added mappings:

VK_SEPARATOR -> PERIOD
VK_MEDIA_PLAY_PAUSE -> MEDIAPLAY
VK_OEM_CLEAR -> CLEAR
VK_PLAY -> MEDIAPLAY
VK_CRSEL -> TAB
VK_ATTN -> ESCAPE
VK_ICO_CLEAR -> CLEAR
VK_ICO_HELP -> HELP
VK_OEM_NEC_EQUAL -> EQUAL

Fixed mappings:

VK_OEM_4 -> BRACKETLEFT
VK_OEM_6 -> BRACKETRIGHT

Documented:
All reserved/unassigned/missing values
Missing IME mappings
The following values:

VK_PROCESSKEY
VK_PACKET
VK_EXSEL
VK_EREOF
VK_ZOOM
VK_NONAME
VK_PA1
VK_OEM_102

Should fix #54958

Thanks to Mozilla's docs for helping me figure out this stuff. :)

[akien-mga]

Superseded by #56514.