Use BreakIterator for search_coll and search_fixed (optionally)

`usearch_open (const UChar *pattern, int32_t patternlength, const UChar *text, int32_t textlength, const char *locale, UBreakIterator *breakiter, UErrorCode *status)` -- a `BreakIterator` may be provided to search only @ specific text boundaries (e.g. match whole words only)

allow `search` funs to be fed with a break iterator to use; add briter support for search_fixed

Locate the usearch_open entry point and the search_coll and search_fixed functions mentioned in the issue. First trace how search currently handles text boundaries, then inspect the BreakIterator parameter and existing briter-related paths. Done means both search functions can optionally use a BreakIterator, including search_fixed.

Written by the indexing model from the issue text.