Separate `overlay.nix` and `overlay.internal.nix`, to encapsulate the
`flake` argument. This allows non-flake users to import `./overlay.nix`
without providing a dummy `flake = null` attr.
The read-only `config.flake` option is only defined by `evalNixvim` when
`lib` is extended using the overlay in Nixvim's flake outputs.
Moved all function-specific docs from `docs/lib/index.md` into RFC145
doc-comments.
Added `lib.nixvim.lua.toLuaObject` to hold the public docs and serve as
a stable impl of `toLua'` in case we decide to change its defaults.