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.

src/nvim/CMakeLists.txt

@@ -953,3 +953,10 @@ add_custom_target(doc-eval DEPENDS ${GEN_EVAL_TOUCH})
 add_custom_target(doc-vim DEPENDS ${VIMDOC_FILES})
 add_custom_target(doc)
 add_dependencies(doc doc-vim doc-eval)
+
+add_custom_target(lintdoc
+  COMMAND ${CMAKE_COMMAND} -E env "VIMRUNTIME=${NVIM_RUNTIME_DIR}"
+    $<TARGET_FILE:nvim_bin> --clean -l scripts/lintdoc.lua
+  WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+  USES_TERMINAL)
+add_dependencies(lintdoc nvim)