Add support for BM25 search

[dimamik]

This pull request introduces BM25 full-text search support to the project, leveraging the pg_textsearch extension for PostgreSQL 17+. It adds the new Torus.bm25/5 macro for state-of-the-art relevance ranking, updates documentation and tests, and ensures the CI pipeline and test environment are compatible with the new requirements. The most important changes are grouped below.

BM25 Full-Text Search Feature:

• Added the Torus.bm25/5 macro in lib/torus.ex for BM25-ranked full-text search, including comprehensive documentation, usage examples, and migration guidance.
• Implemented the BM25 query logic in the new module lib/torus/search/bm25.ex, supporting options like ordering, score selection, pre-filtering, and score thresholding.

Documentation and Guides:

• Updated README.md and CHANGELOG.md to document the new BM25 search capability, its requirements, usage scenarios, and how it compares to existing full-text search. [1] [2] [3] [4]
• Bumped the project version to 0.6.0 in mix.exs and included the changelog in the documentation extras. [1] [2]

CI and Test Environment Updates:

• Upgraded the CI pipeline to use PostgreSQL 17 and Elixir 1.19.5/OTP 28.3, and added installation of the pg_textsearch extension for testing. [1] [2] [3]
• Added a migration for test environments to install pg_textsearch and create BM25 indexes on relevant columns.
• Updated test exclusion tags to support new tests that require CI to be skipped.

Testing and Validation:

• Added compile-time validation tests for the bm25/5 macro to ensure correct option usage, especially around score_threshold and index_name.

[xu-chris]

Thinking Elixir fans jump on for support, brilliant! 🙏
I simply support this effort!