Refactor: describe each supported language once in mkconcore.py instead of repeating if/elif chains #677

Problem: mkconcore.py handles each supported language with separate if langext == ... chains: about ten of them, 56 references to langext in total. They cover runtime-file copying, the Docker build, and the posix/windows/debug script generation. Adding Julia local builds (#669) meant editing about ten places, and #648 (MATLAB literal helper files not copied) is the kind of bug this structure makes easy: one file missing from a hand-written copy list.

Proposal: describe each language once (extension, runtime files for local and docker builds, how to run, how to debug) in a small table, and have the generators loop over it. I would do it in phases, each a small PR that does not change any generated output:

1. the runtime-file copy blocks (the if '<lang>' in required_langs: sections), with a test that every listed runtime file exists,
2. the Docker build selection,
3. the run/debug script generation, one platform at a time.

tests/test_demo_workflows_generate.py (#674) already generates every demo workflow for every build type, so output changes would be caught.

To avoid conflicts, I would start only after the open PRs touching these chains (#669, #637, #639, #635, #672) have been merged or closed.

Does this direction work for you, or would you prefer a different structure? @pradeeban