core: (attr assembly fmt) add qualified directive (pt4)

[n-io]

Add QualifiedParameterVariable (no-op in xdsl since print_attribute always qualifies) and parse_qualified_directive for MLIR format compatibility.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.22%. Comparing base (d122c60) to head (72d4a8a).

Files with missing lines	Patch %	Lines
xdsl/irdl/declarative_assembly_format_parser.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                             Coverage Diff                              @@
##           nicolai/attr-asm-fmt-pt3-optional-groups-v2    #5903   +/-   ##
============================================================================
  Coverage                                        86.22%   86.22%           
============================================================================
  Files                                              418      418           
  Lines                                            60166    60174    +8     
  Branches                                          6977     6978    +1     
============================================================================
+ Hits                                             51878    51885    +7     
  Misses                                            6685     6685           
- Partials                                          1603     1604    +1     

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