ragnar 0.2.0

• ragnar_store_create() gains a new argument: version, with default 2.
  Store version 2 adds support for chunk deoverlapping on retrieval and automatic chunk augmentation with headings.
  To support these features, the internal schema and ingestion requirements are different.
  See markdown_chunk() and new S7 classes MarkdownDocument and MarkdownDocumentChunks.
  Backwards compatibility is maintained with version = 1. (#58, #39, #36)

• ragnar_store_create() now supports Date and POSIXct classes supplied to extra_cols.

• ragnar_store_create() now supports remote MotherDuck Databases specified with md:<dbname> as
  the location argument. (#50)

• ragnar_retrieve() and friends gain a filter argument, adding support for efficiently
  filtering retrieval results.

• ragnar_retrieve_bm25() gains arguments b, k, and conjunctive (#56).

• ragnar_retrieve_vss() gains argument query_vector, supporting workflows that preprocess the query string before embedding.

• ragnar_retrieve_vss() set of valid method choices have been updated to a narrower set to ensure that an HNSW index scan is used.

• Passing a tbl(store) to ragnar_retrieve() is deprecated.

• New chunker markdown_chunk() with support for chunk heading context generation,
  semantic boundary selection, overlapping chunks, document segmentation, and more. (#56)

• New function embed_google_vertex() (@dfalbel, #49)

• New function embed_databricks() (@atheriel, #45)

• New function ragnar_chunks_view() for quickly previewing chunks (#42)

• ragnar_register_tool_retrieve() gains optional name and title arguments
  to allow for more descriptive tool registration. These values can also be set
  in ragnar_store_create() (#43).

• ragnar_read() and read_as_markdown() now accept paths
  that begin with ~ (@topepo, #46, #48).

• Changes to read_as_markdown() HTML conversion (#40, #51):

  • New arguments html_extract_selectors and html_zap_selectors provide a flexible way to
    exclude some html page elements from being included in the converted markdown.
  • code blocks now include the language, if available.
  • Fixed handling of nested code fences in markdown output.