Add ICU sentence segmentation to OffsetsTextInfo (#20603)
Part of #18901.
Follow up for #20602.
### Summary of the issue:
Sentence navigation only works in Microsoft Word and Outlook, which have
native sentence support. Browse mode in virtual buffers uses TextInfos
based on `OffsetsTextInfo`, and its `_getSentenceOffsets` raised
`NotImplementedError`. Working sentence navigation in those documents
therefore needs a sentence backend first.
### Description of user facing changes:
No direct changes. This PR is the necessary building block: programmatic
sentence navigation is now possible in cursor managers and in TextInfos
based on offsets. The next PR in the stack routes the browse mode
sentence gestures to it.
### Description of developer facing changes:
`winBindings.icu` gains `UBRK.SENTENCE`. `textUtils.icu` gains
`calculateSentenceOffsets`. `OffsetsTextInfo._getSentenceOffsets` is
implemented, so `UNIT_SENTENCE` becomes available on all TextInfos
derived from it.
### Description of development approach:
Sentence boundaries come from the Windows built-in ICU BreakIterator
using UAX#29 default rules. Segmentation runs over the containing
paragraph, so sentences can span lines. Results are rebased by the
paragraph start offset. The root locale is used: UAX#29 sentence
boundaries are driven by the language-neutral Sentence_Break property,
and locale only adds abbreviation tailoring. A fixed locale also keeps
boundaries a deterministic function of position, which the
`move`/`expand` tiling invariant requires. When ICU is unavailable
(Windows before version 1703), the method keeps raising
`NotImplementedError`.
### Testing strategy:
New unit tests in `test_sentenceSegIcu.py` cover the
`calculateSentenceOffsets` primitive (English, Japanese, abbreviation
splits, surrogate pairs, containment) and the `OffsetsTextInfo`
integration, including forward and backward tiling across a
block-paragraph fixture.
### Known issues with pull request:
Abbreviations such as "Dr." end a sentence. Locale-specific tailoring
could address this later.
`documentNavigation.sentenceHelper` still uses its own logic to detect
sentence boundaries.
