[utils] importLua function and luaToNix script

[MattSturgeon]

It'd be useful when adding/updating plugin modules to be able to paste some lua code into a script and get out valid nix. Effectively the opposite of helpers.toLuaObject.

This may also be useful within nixvim modules, either internally or for users.

It'd be nice to have a pure-nix function to do the heavy lifting, and then expose this through a script or something for use on the CLI. This would also apply to toLuaObject.

[MattSturgeon]

Actually, I'm tempted to move toLuaObject out of tree into a dedicated nix/lua tools flake.