[code-infra] Add types for markdown loader

[Janpot]

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Added a few types to better under stand how the code flows wrt webpack cache. May as well commit them for future use. The code didn't alter semantically in any way, it's 100% types.

@dav-is I remember you mentioned before that this loader doesn't do correct dependency tracking. I'm seeing addDependency all over the place, did you have a specific case in mind?