support type checking html files

[scarf005]

currently ESM script inside html files aren't typechecked. only external js will be typechecked. however, being able to typecheck them will be valuable in editing single-file project, since for non single-file project (that uses <script type="module" src="...">), there's these shortcomings:

• import maps has to be duplicated between index.html and deno.json: External import maps support WICG/import-maps#235
• importing external ESM module requires file server due to CORS