feat: Allow emission of diagnostics inside GuppyError

[mark-koch]

Closes #538.

• Allow to pass a Diagnostic object to GuppyError instead of an error message
• The old error reporting system remains untouched until we have migrated everything to the new diagnostics system
• Adds a TypeMismatchError diagnostic as a test to ensure that the pipeline works

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.22%. Comparing base (f9709ae) to head (6d3eef7).

Files with missing lines	Patch %	Lines
guppylang/checker/expr_checker.py	86.66%	2 Missing :warning:

Additional details and impacted files

@@                 Coverage Diff                  @@
##           feat/diagnostics     #553      +/-   ##
====================================================
+ Coverage             91.09%   91.22%   +0.12%     
====================================================
  Files                    61       61              
  Lines                  6482     6505      +23     
====================================================
+ Hits                   5905     5934      +29     
+ Misses                  577      571       -6     

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