Add publish keyword

[georg-schwarz]

Partially addresses #563: Introduces publish keyword.

• Extended the grammar with the keyword
• Adjusted scoping to only make published elements available in other files
• Added validation to avoid cyclic imports
• Published all builtin elements
• Adapted tests

As discussed in eclipse-langium/langium#1508 (reply in thread), the main logic resides in the JayveeScopeProvider instead of the JayveeScopeComputation.

Notes

• Decide whether we should add caching (as suggested in the thread) right away or track it in a separate issue: already implemented, only 3 lines of code.
• Validate code actions of Import Code Actions #565 still work
• Validate that cyclic dependencies are prevented
• Add user documentation

[georg-schwarz]

If we should ever allow publish * from '{path}', we'd have to use recursion here I think to follow the chain.. Right now, the exported element is directly referenced.

[rhazn]

Had a high level look, overall looks good to me, great work 👍 . Just some small codestyle / optional comments.