psi: optimize AST traversal and fix tree-sitter resource leaks

[wenxuanjun]

This PR implements performance optimizations for the indexing process and fixes memory leaks throughout the LSP lifecycle.

Refactored build_stub_tree to use a lightweight TreeWalker with manual cursor management, avoiding unnecessary object creation.

Addressed memory leaks where Tree-sitter C-side resources were not being freed. I also ensures temporary parsers used during constant and enum value evaluation are correctly freed.

Resolves #131.

Benchmarks

Indexing a project (including vlib and vmodules) without stub cache:

Dev build:

• Before: 10.75s, 72MB peak memory
• After: 7.96s, 50MB peak memory

Prod build:

• Before: 6.9s, 63MB peak memory
• After: 5.6s, 48MB peak memory

Test suite (v test src/tests):

• Before: 7102.553 ms
• After: 3324.785 ms

[spytheman]

Excellent work.
Thank you for working on it 🙇🏻 .
A lot.