Add basic Cursor AI .cursorignore File

[holgerd77]

This PR adds a basic Cursor .cursorignore file which exludes large code parts (mainly the ethereum-tests submodule directory) to allow Cursor for sufficiently performant code base indexing, this now takes < 5 min first time for the whole monorepo codebase.

Indexing gives Cursor a better understanding of the code base and provides better results on AI prompts.

I have purposefully left the new execution-spec-tests submodule directory in since I think this might be helpful for Cursor as context since there is mainly "the current stuff" integrated. 😄

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.31%. Comparing base (6011468) to head (950bb2c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
block	84.33% <ø> (ø)
blockchain	89.32% <ø> (ø)
common	97.51% <ø> (ø)
evm	72.77% <ø> (ø)
mpt	89.74% <ø> (-0.31%)	⬇️
statemanager	69.06% <ø> (ø)
static	99.77% <ø> (ø)
tx	89.89% <ø> (ø)
util	88.71% <ø> (ø)
vm	55.31% <ø> (ø)

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