Migration of Epoch into separate repository

[a-sr]

This PR factors out all Eclipse (Epoch) specific projects and restructures the remaining source files such that they can be used as a sub module. The new repository is here: https://github.com/lf-lang/epoch
Implements parts of #1276.

Involved tasks (LF repo):

• Move contents of org.lflang.diagram and org.lflang.ide to org.lflang
• Remove all Epoch related projects
• Move EclipseErrorRepoter to Epoch repo
• Adjust mwe build configuration
• Document mwe build configuration implications (@a-sr)
• Adjust Gradle build
• Adjust GH build actions
• Fix Gradle tests

Involved tasks (Epoch repo):

• Include lingua-franca repository as submodule
• Adjust Maven build
• Adjust mwe build configuration
• Setup GH build actions
• Adjust Oomph setup
• (not possible) Reduce redundancy in mwe configurations
• Fix eclipse test (template check)
• Implement Epoch in-tool compilation test
• Set repo to public

Remaining tasks:

• Update path to jar vscode-lingua-franca#100
• Fix the nightly build #1613
• CI check for compatibility with Epoch #1614
• Update README.md to describe this repo epoch#3
• Remove Oomph docs lf-lang.github.io#128

[a-sr]

@cmnrd, @lhstrh I'd say the bulk of the work for the restructuring is done now. The Gradle build in LF works and likewise does the maven build in the new Epoch repo. Everything is as before, just in separate repos.
I still have a few open ToDos (see PR description) and I would appreciate if you could help me with these.
My current focus is on wrapping things up in the new Epoch repo. Hence, it would be great if one of you could take a look at the failing Gradle tests for this branch. After I moved the diagrams and ide plugins into org.lflang, suddenly the majority of JUnit test run by Gradle reported a SecurityException. I did not yet further investigated the stacktrace.

Btw, the build job in GH only fails for this PR because is uses the configuration from the master branch. Since I only removed the maven build commands, the GH configuration should be fine.

[lhstrh]

This looks like fantastic progress, @a-sr! Thank you so much for doing this. It looks soooo much cleaner. I can pitch in with the remaining work. Perhaps @petervdonovan could with the adjustments that will be necessary in the VS Code repo.

[lhstrh]

This all looks great! Thanks again for doing this. Let's hold off the merge for now, but I'll make it a priority to get it merged very soon.

[petervdonovan]

This branch deletes the generateLanguageDiagramServer Gradle task. How am I supposed to build the language server now?

Edit: Sorry for the noise, I understand what happened now. I will submit a PR in vscode-lingua-franca to bring it up to date.

[lhstrh]

Is it lf-lang/vscode-lingua-franca#100?