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.
