Introduce a bundle with dependencies for running MWE2 workflow

[LorenzoBettini]

Closes #3208

A few notes:

• the new bundle replaces the dependencies we used to generate as additional.bundles in build.properties; it has to re-export such dependencies so that the mwe2 workflow finds what it needs to complete the execution
• the examples have been updated accordingly
• other build.properties, e.g., some of our test projects containing languages, have NOT been updated because the additional.bundles were less, so I found it useless
• I haven't updated the POM files generated from the wizard, for plain Maven projects and for Maven/Tycho projects, because I seem to understand that is not necessary:
  • mwe2 dependencies must be available to make the mwe2 file usable in the IDE, so in a plain Maven project they must be dependencies of the project, not of the exec-maven-plugin configuration, as they are, and in an Eclipse plug-in project, additional.bundles are enough
  • Maven/Tycho uses the additional.bundles in the current exec-maven-plugin configuration that we generate with the wizard, the only additional dependency of the exec-maven-plugin is the custom antlr generator that must be a Maven dependency, since we cannot deploy it in an Eclipse update site due to licence problems
  • summarizing, the new bundle would not make sense as a Maven dependency of the exec-maven-plugin

Maybe, for the above reasons, a better name could be org.eclipse.xtext.xtext.generator.additional.bundles for this new bundle, in case the current name does not sound correct. It would not even be required to deploy it as a Maven artifact, but it doesn't cost us anything to do that.

[github-actions]

Test Results

  6 460 files  ±0    6 460 suites  ±0   3h 2m 7s ⏱️ - 1m 14s
 43 242 tests ±0   42 658 ✅ +1    584 💤 ±0   0 ❌  - 1 
170 270 runs  +2  167 898 ✅ +1  2 358 💤 ±0  14 ❌ +2 

Results for commit a54b442. ± Comparison against base commit dcfe810.

♻️ This comment has been updated with latest results.

[LorenzoBettini]

I rebased. When the build finishes I'll merge