helpers: Add option helpers for types that are either a lua str or a value (#876)

Those types needed the following code in apply:

if builtins.isString value
then mkRaw value
else value

This commit avoids this boilerplate, and clarifies that the `str` is lua
code in the documentation.

plugins/dap/dap-python.nix

@@ -37,7 +37,7 @@ in {
       If neither is found "unittest" is used.
     '';
 
-    testRunners = helpers.mkNullOrOption (types.attrsOf types.str) ''
+    testRunners = helpers.mkNullOrOption (with helpers.nixvimTypes; attrsOf strLuaFn) ''
       Set to register test runners.
       Built-in are test runners for unittest, pytest and django.
       The key is the test runner name, the value a function to generate the