core: Factor BaseParser out of the parser (NFC)

[math-fehr]

Currently, parser/core.py is a huge file that should be factored more.
This first PR factors out BaseParser out of that file, which contains the mains methods that are used in the different parsers we will have (AffineParser, BuiltinAttrParser, AsmParser).

Factoring this also helps understanding what base functions are available.

This PR only moves methods, and improve a bit on some documentation, but has no functional change.

[codecov]

Codecov Report

Patch coverage: 95.16% and no project coverage change.

Comparison is base (3b930bf) 88.75% compared to head (6cc233c) 88.76%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1191   +/-   ##
=======================================
  Coverage   88.75%   88.76%           
=======================================
  Files         161      162    +1     
  Lines       22455    22458    +3     
  Branches     3409     3411    +2     
=======================================
+ Hits        19931    19934    +3     
  Misses       1981     1981           
  Partials      543      543           

Impacted Files	Coverage Δ
xdsl/parser/base_parser.py	94.87% <94.87%> (ø)
xdsl/parser/affine_parser.py	90.41% <100.00%> (+0.13%)	⬆️
xdsl/parser/core.py	83.22% <100.00%> (-2.35%)	⬇️

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[webmiche]

tests pass, so I'm fine with this 👍