Add spell check TextSpan creation logic that doesn't rely on composing region (#123481)

Fixes https://github.com/flutter/flutter/issues/119534 by adding extra
logic to allow the proper `TextSpan` trees to be build with spell check
results that does not rely on the composing region.

Also, refreshes the algorithm used to correct spell check results to
improve cases where spell check results are out of date, but the
composing region line does not help smooth over this fact by giving the
framework extra time to update while a word is being composed (because a
composing region line would cover up any flashing red underlines).

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] All existing and new tests are passing.

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