Add invariant assertion for AST

[fwendland]

We want the ability to check invariants for an AST (abstract syntax tree). These invariants are a practical mean to ensure that new features don't break our specification of the CPG.

For the AST our invariants are (subject for discussions):

• tree structure
• for every node, every node.child.parent is node
• ...

This PR supplies an initial proposal.

We add the invariant assertions as separate function to be called on demand.

Open integration:

• Add to analyze() conditional to a flag (default: false)

[codecov]

Codecov Report

❌ Patch coverage is 52.94118% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.02%. Comparing base (e72a491) to head (285d9b1).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...s/kotlin/de/fraunhofer/aisec/cpg/test/TestUtils.kt	52.94%	7 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
...s/kotlin/de/fraunhofer/aisec/cpg/test/TestUtils.kt	67.28% <52.94%> (-3.99%)	⬇️

... and 1 file with indirect coverage changes

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