Rewrite lua_api.txt into Markdown format

This PR makes a huge update to lua_api.txt by rewriting it into Markdown format entirely. Previews have been tested with two parsers (including Dingus) and it looked alright with both.

I just used Markdown, no variant of it. File name intentionally not changed to “lua_api.md” because of fear to break references to that file. The file can still be renamed in a later point of time.