Writing Flow: Compute caret DOMRect on-demand for vertical traversal

[aduth]

Fixes #15604
Regression of #13697

This pull request seeks to resolve an issue where, when vertically navigating between paragraphs using ArrowUp, ArrowDown, the caret does not always land where would be expected to preserve its horizontal position.

Implementation notes:

See exhaustive debugging detail at #15604 (comment).

Testing instructions:

Repeat Steps to Reproduce from #15604, verifying expected result.

Ensure end-to-end tests pass:

npm run build && npm run test-e2e packages/e2e-tests/specs/writing-flow.test.js

Verify ArrowUp, ArrowDown vertical traversal works generally well in a variety of circumstances.

[aduth]

@youknowriad We might want to consider this one for backport (WordPress 5.2.1), as it does appear to afflict WordPress 5.2.0.

[ellatrix]

Will caret position reset when you click somewhere else? I'll test and review in a bit.

[ellatrix]

This won't work correctly in the following case, which should probably be added to the e2e tests:

Type "1", press Shift+Enter, press Enter, type "2". Place the caret after "1". Press ArrowDown twice. The caret should appear after "2".

In other words, the initial caret position must be preserved between arrow key presses to the closest position to the initial one can be set. It must be unset after mousedown or arrow left/right. It looks like it is only unset on mousedown. Edit: it is.

[aduth]

Closing in favor of #15624