Add Solidity symbol and graph extraction

[Widthdom]

Summary

• Add Solidity symbol extraction for contracts, interfaces, libraries, constructors, functions, events, errors, structs, enums, modifiers, fallback, and receive declarations.
• Add conservative Solidity graph references for inheritance, using ... for library usage, modifier applications, event emissions, and interface-style calls.
• Update language capability docs/tests and add an unreleased changelog fragment.

Fixes #3531

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~Solidity
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~RunLanguages_Json_SearchOnlyBucketsAdvertiseZeroSymbolAndGraphSupport
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format --verify-no-changes --no-restore
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Notes

• Full dotnet test was attempted, but the VSTest/testhost process stayed alive for several minutes without output and was stopped after targeted tests passed.