[web] Changes to EditableState to be able to handle Framework text selection shortcuts

[htoor3]

Currently there's an issue in which keyboard shortcuts (shift + arrow keys) don't autoscroll on web inputs and textfields. Delegating these keyboard actions to the framework fixes this issue and allows for a more consistent UX for web.

As part of that change, the EditableState class needs to be able to handle flipped base and extent offsets, as well as have sensible defaults for composing offsets.

Part 1 fixes flutter/flutter#107707

Framework changes PR: flutter/flutter#114264

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

[mdebbar]

Great tests!

Let's also add one to make sure EditingState.fromFrameworkMessage() sets the composing values to -1.

Could you also open the framework-side PR so reviewers can look at both PRs together?

[htoor3]

Great tests!

Let's also add one to make sure EditingState.fromFrameworkMessage() sets the composing values to -1.

Could you also open the framework-side PR so reviewers can look at both PRs together?

@mdebbar Added that suggested tests and the PR in the description

[mdebbar]