Add simple Importer from Forge 5.x to 6.x

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Adds support for changing config.forge if it doesn't point to a JS file. In the future we could possibly provide updateForgeConfig for those JS files.

There was some refactoring involved - as a result, getElectronVersion has test coverage.

In general, the feature was written in such a way that it should have complete unit test coverage from a LoC perspective.

[malept]

Not sure why Tidelift is failing. It may just need a commit to master to kickstart it.

[malept]

Electron nightly support was added to this PR because I was already in readElectronVersion.

[MarshallOfSound]

Seems legit

[malept]

😆 I need to pick a better electron version in the tests