* refactor(native_api_header): upgrade header files to Lua 5.4.

Almost all of the symbols in this file was from 5.2. This will obviously
not work because some function signatures have changed and some have
completely wrong return values, etc.
This commit updates the header files to Lua 5.4 based on the source code
and changes a few things.

* refactor(plugin_api): move the header into include/

* fix(lite_xl_plugin_api.h): include stdlib to avoid errors with exit

* refactor(lite_xl_plugin_api.h): do not return in SYMBOL_WRAP_CALL

* fix(lite_xl_plugin_api.h): fix wrong way of passing varargs

* fix(lite_xl_plugin_api.h): fix differing lua_rawlen definition

* fix(lite_xl_plugin_api.h): fix fallback function signature

* fix(lite_xl_plugin_api.h): fix conversion from void * to function pointer