Add KeyEventResult enum and support passthrough (filter) mode

[buzztaiki]

This PR adds support for a passthrough mode to the KeyEvent handler by extending its return value handling.

I also introduced the KeyEventResult enum to provide a way to choose between NotHandled, Handled, and Passthrough from Lua. This solves the issue where users need to bypass Fcitx5 for specific applications (like Emacs with DDSKK) in Wayland environments.

Example usage

local fcitx = require("fcitx")

local target_programs = {
   ["emacs"] = true
}

function handler(keysym, states, release)
   local program = fcitx.currentProgram()
   if target_programs[program] then
      return fcitx.KeyEventResult.Passthrough
   end
   return fcitx.KeyEventResult.NotHandled
end

fcitx.watchEvent(fcitx.EventType.KeyEvent, "handler")

[wengxt]

can you add a test?

[buzztaiki]

sure, I try to add a test.

[buzztaiki]

@wengxt I've added tests for KeyEvent result.