XRM editor syntax serialization for mapping described in RDF

[mchlrch]

Adding the possibility to serialize an RDF representation (see #127) of xrm mapping elements (everything: source, vocab, mapping, dialect, ...) back to xrm editor syntax.

This improves hackability and unlocks possibilities for toolchain improvements outside of the xrm editor itself, for example:

• experimentation with new kinds of mapping UIs
• experimentation with auto-generated mappings ( direct mappings, LLMs, ...) followed by post-processing in xrm editor

From the outside, this looks like integration on editor syntax level. Before implementing this, we also need to consider the option of first introducing a domainmodel layer into xrm, as this would yield additional extension points for toolchain integration independent of the xrm editor.