Add popmark support to the code snippets in dart_skills_lint

[reidbaker]

The use it to pull out code that is marked as dart an evaluate it.

[reidbaker]

This wound up being a bad idea. Code samples were in practice a mix of code that needed to be in a main or full imports and the logic for configuring templates added complexity that looked gross.

[reidbaker]

If we wanted to introduce node as a requirement (which i dont) then syntax validation with prettier (nvm prettier does not support dart) would be nice or using eslint-plugin-markdown (js) or pytest-codeblocks(python) would be good. Final investigation is if there is code for the dart website that could be published on its own.

Attempts to use analyzer parseString failed because it throws for any finding including unused variables which made it a non starter.