deno test --doc should be able to typecheck Markdown files

[bartlomieju]

deno test --doc should typecheck codeblocks in Markdown files by default.

It will make it easier to keep documentation up to date, especially keeping Deno manual examples working on each commit.

Doing so will keep it inline with deno fmt and deno lint which can already act on Markdown files.

CC @caspervonb

[caspervonb]

I've implemented this in #11421 but... we do make use of a special marker STD_VERSION which means these fenced code blocks aren't currently valid so we need to either remove that or work around it somehow without making the test runner even more complicated.