lib/util: move docs from lib/index to doc-comments

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.

docs/lib/index.md

@@ -65,47 +65,3 @@ This can be achieved using the lib overlay, available via the `<nixvim>.lib.over
     };
 }
 ```
-
-## Common helper functions
-
-A certain number of helpers are defined that can be useful:
-
-- `helpers.emptyTable`: An empty lua table `{}` that will be included in the final lua configuration.
-  This is equivalent to `{__empty = {};}`. This form can allow to do `option.__empty = {}`.
-
-- `helpers.mkRaw str`: Write the string `str` as raw lua in the final lua configuration.
-  This is equivalent to `{__raw = "lua code";}`. This form can allow to do `option.__raw = "lua code"`.
-
-- `helpers.toLuaObject obj`: Create a string representation of the Nix object. Useful to define your own plugins.
-
-- `helpers.listToUnkeyedAttrs list`: Transforms a list to an "unkeyed" attribute set.
-
-  This allows to define mixed table/list in lua:
-
-  ```nix
-    (listToUnkeyedAttrs ["a" "b"]) // {foo = "bar";}
-  ```
-
-  Resulting in the following lua:
-
-  ```lua
-    {"a", "b", [foo] = "bar"}
-  ```
-
-- `helpers.enableExceptInTests`: Evaluates to `true`, except in `mkTestDerivationFromNixvimModule`
-  where it evaluates to `false`. This allows to skip instantiating plugins that can't be run in tests.
-
-- `helpers.toRawKeys attrs`: Convert the keys of the given `attrs` to raw lua.
-  ```nix
-    toRawKeys {foo = 1; bar = "hi";}
-  ```
-  will translate in lua to:
-  ```lua
-    {[foo] = 1, [bar] = 2,}
-  ```
-  Otherwise, the keys of a regular `attrs` will be interpreted as lua string:
-  ```lua
-    {['foo'] = 1, ['bar'] = 2,}
-    -- which is the same as
-    {foo = 1, bar = 2,}
-  ```