feat: add signature help

[sophiathedev]

Summary

• expose ElixirSense signature data through the project Engine API
• handle textDocument/signatureHelp with active parameters, documentation, specs, and selected metadata
• advertise ( and , as signature-help trigger characters

Testing

• Engine targeted tests: 3 passed
• Expert targeted tests: 3 passed
• Engine full suite: 1164 passed, 19 excluded
• Expert full suite: 844 passed, 12 excluded
• mix compile --warnings-as-errors, mix format --check-formatted, mix credo --strict, and mix dialyzer in both applications

Closes #142