Implement and test optional operand and result variables.

[PapyChacal]

Stacked on #2106.

Add optional operands and type directives; very similar to the variadic ones. Adds the notion of "variadic-like" to relate to checks added in #2106

[codecov]

Codecov Report

Attention: Patch coverage is 93.66197% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 89.50%. Comparing base (51461d6) to head (a7723af).

Files	Patch %	Lines
xdsl/irdl/declarative_assembly_format.py	92.75%	2 Missing and 3 partials ⚠️
xdsl/irdl/declarative_assembly_format_parser.py	85.18%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2107      +/-   ##
==========================================
+ Coverage   89.48%   89.50%   +0.01%     
==========================================
  Files         326      326              
  Lines       38873    38983     +110     
  Branches     5754     5773      +19     
==========================================
+ Hits        34787    34890     +103     
- Misses       3288     3292       +4     
- Partials      798      801       +3     

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[math-fehr]

Very very nice, thanks a lot <3