core: (attr assembly fmt) add params directive (pt5)

[n-io]

Add ParamsDirective that captures all parameters of an attribute and prints them comma-separated. Includes _all_param_variables helper in AttrFormatParser, with tests for basic, three-param, and optional cases.

[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.22%. Comparing base (72d4a8a) to head (446f866).

Files with missing lines	Patch %	Lines
xdsl/irdl/declarative_assembly_format_parser.py	76.92%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@                          Coverage Diff                           @@
##           nicolai/attr-asm-fmt-pt4-qualified-v2    #5904   +/-   ##
======================================================================
  Coverage                                  86.22%   86.22%           
======================================================================
  Files                                        418      418           
  Lines                                      60174    60214   +40     
  Branches                                    6978     6988   +10     
======================================================================
+ Hits                                       51885    51922   +37     
- Misses                                      6685     6687    +2     
- Partials                                    1604     1605    +1     

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