test: overhaul of testing infra and add more tests

[pmmmwh]

This PR adds:

1. Infrastructure to test code related to Webpack compilation;
2. Tests related to loader code w.r.t Webpack 4/5 utilizing (1);
3. Unit tests for any non-trivial functions, to increase test coverage and confidence in reliability of the plugin.

In the future, with most testing infra stabilized, we can add more cases related to the error overlay and the error recovery experience (see #3 for cases).