Writing Flow: Avoid horizontal writing flow navigation in native inputs

[aduth]

Closes #6468

This pull request seeks to update the writing flow behavior to allow only unmodified vertical arrow key presses for consideration in a browser native input (textarea, input).

Testing instructions:

Verify that horizontally navigating between non-input fields still works.

1. Insert two paragraph
2. At the beginning of the second paragraph, press ArrowLeft
3. Ensure that focus is placed at the end of the first paragraph

Verify that only unmodified vertical arrow press in native input transitions focus:

1. Click editor title
2. Press ArrowRight
3. Observe no focus change
4. Press Ctrl/Cmd + ArrowDown
5. Observe no focus change
6. Press ArrowDown
7. Ensure that focus is placed in a new default paragraph

Ensure unit tests and end-to-end tests pass:

npm run test-unit

npm run test-e2e

[jorgefilipecosta]

Implements the desired behaviour in my tests 👍

[jorgefilipecosta]

Would it make sense to apply this to all the editables?

[aduth]

Can you elaborate on what you mean? Are you thinking to reverse this condition so that we test whether the candidate is explicitly contentEditable ? Or do you mean we should include editables In the excluded set of candidates? That would cause issues for navigating between paragraphs, where we do want ArrowRight to navigate between blocks.