Remarkup code blocks: guess language from "name="

Summary:
The file name is a sufficient source of information, if available.

Closes T15729

Test Plan: The new unit test is green. Old unit tests are green.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15729

Differential Revision: https://we.phorge.it/D25560