Index modules with RBS indexer

[alexcrocha]

One more step towards #87.

This PR starts indexing modules with the RBS indexer.

• .rbs files are now collected during file discovery (previously, only .rb files were collected)
• Added new_rbs() to LocalGraphTest so rbs_indexer tests can use the same assertion pattern as ruby_indexer
• Added index_rbs_uri() to GraphTest as a separate method from index_uri() due to in-memory sources
• Extracted shared test macros from ruby_indexer to local_graph_test.rs so both indexers can reuse them
  • Diagnostic macros were renamed to assert_local_diagnostics_eq! / assert_no_local_diagnostics! to avoid conflicting with the GraphTest versions in graph_test.rs

[alexcrocha]

• Index modules with RBS indexer #591 👈 (View in Graphite)
• main

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