refactor(lua): call loadfile internally instead of luaL_loadfile

[lewis6991]

A version of loadfile with the intention of being redefined to allow third-party plugins to provide instrumentation and caching.

This will allow impatient.nvim to cache Lua files in directories like plugin and after.

loadfile is still called via C so the only additional overhead is via the Lua-C API which shouldn't be too bad.

Obviously this will need some discussion (hence this is a draft), but I hope we agree on the overall goal.

EDIT: Changed so the Lua built-in loadfile is called instead of the direct C version luaL_loadfile.

[lewis6991]

Before I work on this any more, @bfredl are you OK with the premise of this PR?

[bfredl]

@lewis6991 yes.

[bfredl]

Needs an update to tests:

2022-02-02T18:10:27.5497661Z test/helpers.lua:73: Expected objects to be the same.
2022-02-02T18:10:27.5497930Z Passed in:
2022-02-02T18:10:27.5498383Z (string) 'Vim(luafile):E5113: Error while calling lua chunk: attempt to call a nil value
2022-02-02T18:10:27.5498725Z stack traceback:'
2022-02-02T18:10:27.5498958Z Expected:
2022-02-02T18:10:27.5499559Z (string) 'Vim(luafile):E5112: Error while creating lua chunk: Xtest-functional-lua-commands-luafile:1: unexpected symbol near ')''

I haven't checked the test in detail but likely just putting in the new message should be fine.