rename some things to be more pico8ish

[Shadowblitz16]

warning -> warn
setfps -> fps
_keyup -> keyr
_keydown -> keyp
_textinput -> keyc
_touchup -> touchr
_touchdown -> touchp
_getcursorx -> curx
_getcursory -> cury
_getpicoloveversion -> ver

[Shoozza]

While the names could be shorter, they probably should still start with _ to indicate they are not compatible with pico-8.

[Shoozza]

It might be good to only enable them when included.

Something like #include picolove-extensions.lua.