fix(windows): modifer event is always serialized

PR #7449 added modifier event to make ensure modifer events
where sent to the serializer even if the key stroke was determined
the key stroke did not need seralizing. However, a logic error was made
trying to use the exiting 'Flag' however this was not set in the
!UseCachedHotkeyModifierState case. This change adds helper function
to simply check if the key is a modifier key and send it to the
event seralizer in that case. Separating from the other logic which
is more concerned with handling hotkeys.