Mypy: fix helper functions

[jstucke]

supersedes #1123 and #1142

• added mypy config to pyproject.toml
• fixed mypy type issues in all modules in helperFunctions

this does not include the pre-commit hook yet (otherwise all other builds would fail after merging this PR)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.29412% with 4 lines in your changes missing coverage. Please review.

Project coverage is 91.86%. Comparing base (42f7320) to head (eb19521).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
src/helperFunctions/plugin.py	77.77%	2 Missing ⚠️
src/helperFunctions/fileSystem.py	0.00%	1 Missing ⚠️
src/objects/file.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1224      +/-   ##
==========================================
- Coverage   91.96%   91.86%   -0.11%     
==========================================
  Files         377      376       -1     
  Lines       22360    20952    -1408     
==========================================
- Hits        20564    19248    -1316     
+ Misses       1796     1704      -92     

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