Add TextInput hook

With !1952 (merged) being queued up for 2.2.14, it would only be logical to expose that API to Lua, so scripters can take advantage of this, too. This also complements KeyDown and KeyUp hooks, but due to technical reasons around dead keys and such, TextInput won't be invoked unless input.setTextInputMode is called first (this is also true for the engine; there is no exception for Lua here).

Example script:

addHook("MapLoad", function (mapnumber)
  input.setTextInputMode(true)
end)
addHook("TextInput", function (textevent)
  print(textevent.text)
end)