feat(core): add Solidity file support

[Showiix]

Summary

• include .sol in the default supported file extensions
• map .sol files to the solidity language so the existing LangChain splitter support is used
• add a regression test that indexes a Solidity file without custom extensions
• update default extension docs

Tests

• pnpm --filter @zilliz/claude-context-core test -- context.splitter.test.ts
• pnpm --filter @zilliz/claude-context-core test
• pnpm --filter @zilliz/claude-context-core typecheck
• pnpm --filter @zilliz/claude-context-core build
• git diff --check

Fixes #240