feat(core): add Elixir language support

[txhno]

Summary

Adds default Elixir file support to the core indexer.

• include .ex and .exs in the default supported extension list
• map both extensions to the elixir language before splitting
• add a focused indexing test for Elixir files

Fixes #315

Validation

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