build(docs): separate lint job to validate vimdoc #27227

Summary: Separate the lint job (`make lintdoc`) to validate runtime/doc,
it is no longer as a part of functionaltest (help_spec).

Build (cmake) and CI:

- `make lintdoc`: validate vimdoc files and test-generate HTML docs.
  CI will run this as a part of the "docs" workflow.

- `scripts/lintdoc.lua` is added as an entry point (executable script)
  for validating vimdoc files.

scripts/gen_help_html.lua:

- Move the tests for validating docs and generating HTMLs from
  `help_spec.lua` to `gen_help_html`. Added:
  - `gen_help_html.run_validate()`.
  - `gen_help_html.test_gen()`.

- Do not hard-code `help_dir` to `build/runtime/doc`, but resolve from
  `$VIMRUNTIME`. Therefore, the `make lintdoc` job will check doc files
  on `./runtime/doc`, not on `./build/runtime/doc`.

- Add type annotations for gen_help_html.

Makefile

@@ -117,7 +117,7 @@ functionaltest-lua: | nvim
 	$(BUILD_TOOL) -C build functionaltest
 
 FORMAT=formatc formatlua format
-LINT=lintlua lintsh lintc clang-analyzer lintcommit lint
+LINT=lintlua lintsh lintc clang-analyzer lintcommit lintdoc lint
 TEST=functionaltest unittest
 generated-sources benchmark $(FORMAT) $(LINT) $(TEST) doc: | build/.ran-cmake
 	$(CMAKE) --build build --target $@