fix(windows): Handle Caps Lock event correctly from TIP

Fixes #4525.

This is a regression from #4468, which inadvertently added modifier
checks for `VK_CAPITAL` and `VK_NUMLOCK`, when it should not have done
so, as the flag was then being reset when the key was released rather
than toggled.

`ProcessModifierChange` pretended that it could handle `VK_CAPITAL` and
`VK_NUMLOCK` but it would never have worked, and the other caller (the
GetMessage hook) never passed those key events through, only Shift, Ctrl
and Alt, so that's now what the TIP handler does as well.

This leaves the rest of the fix from #4468 in place as that appears
correct.

I took the opportunity to move a repeated function declaration into
keyman64.h.