core: Implement first cases of assembly format optional groups

[PapyChacal]

This covers optional groups with no else group, only literal first then element, and optional/variadic operand/result/type anchors. I think this is the most widely used, it enables the typical (`(` $variadic_operand`)`)?

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 89.49%. Comparing base (d2fa75d) to head (300f5b4).
Report is 2 commits behind head on main.

Files	Patch %	Lines
xdsl/irdl/declarative_assembly_format.py	94.02%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2239      +/-   ##
==========================================
+ Coverage   89.47%   89.49%   +0.02%     
==========================================
  Files         328      328              
  Lines       39235    39361     +126     
  Branches     5811     5837      +26     
==========================================
+ Hits        35104    35227     +123     
- Misses       3323     3327       +4     
+ Partials      808      807       -1     

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