Suggestion: move the --doc option to the deno check command

[GJZwiers]

deno test includes the --doc option which enables type-checking JSDoc and Markdown code blocks. I think it would make sense to move the option to deno check and enable type-checking documentation with deno check --doc. It would separate concerns, making deno test responsible for testing code and deno check responsible for type-checking code. deno check --doc would also convey more meaning than deno test --doc from a semantic perspective.