(towards #3083) Scaffolding for Treesitter frontend

[sergisiso]

This PR provides a barebones treesitter frontend that allows to do: psyclone --frontend treesitter test.f90 to essentially generate a treesitter codeblock encompassing the whole file. This needed a few changes because the CodeBlock and Reader only had one concrete class each and fparser was leaking in many places.

This PR:

• Creates a treesitter reader
• Encapsulates parser generation functionality inside the respective reader files (fparser and treesitter)
• Subclass CodeBlock for fparser and treesitter

The performance of treesitter won't be ideal until all leaks of fparser are container (e.g. it is only lazy loaded from inside its reader and codeblock), because a large part of the fparser performance hit is at import time.

Hopefully follow on PRs can be more contained inside the reader class.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (c54c5f1) to head (983b205).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #3351    +/-   ##
========================================
  Coverage   99.95%   99.95%            
========================================
  Files         387      388     +1     
  Lines       54315    54432   +117     
========================================
+ Hits        54293    54410   +117     
  Misses         22       22            

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