[WIP] core: migrate xdsl-opt targets to Multiverse

[superlopuh]

I would like xdsl-opt to be extensible via plugins for targets, similarly to today's dialects and passes. This will bring us one step closer to avoiding opt-like tools in dependent frameworks, for things like codegen, analysis dumps, and interpretation.

Opening this now as a draft PR to make the final state available as I pull out individual changes to review. (should be one PR per commit in this branch)

[codecov]

Codecov Report

❌ Patch coverage is 95.11494% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.37%. Comparing base (583e55b) to head (6679014).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
xdsl/utils/arg_spec.py	93.79%	4 Missing and 4 partials ⚠️
xdsl/targets/riscemu.py	86.36%	3 Missing ⚠️
xdsl/dialects/wasm/wat.py	87.50%	1 Missing and 1 partial ⚠️
xdsl/backend/wgsl/wgsl_printer.py	92.30%	0 Missing and 1 partial ⚠️
xdsl/utils/exceptions.py	50.00%	1 Missing ⚠️
xdsl/utils/target.py	91.66%	0 Missing and 1 partial ⚠️
xdsl/xdsl_opt_main.py	96.15%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5702      +/-   ##
==========================================
+ Coverage   86.34%   86.37%   +0.02%     
==========================================
  Files         406      410       +4     
  Lines       57732    57885     +153     
  Branches     6707     6717      +10     
==========================================
+ Hits        49848    49997     +149     
- Misses       6325     6326       +1     
- Partials     1559     1562       +3     

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