Restore title separators focus style

[afercia]

Summary

This PR can be summarized in the following changelog entry.

• Restore focus indication on the title separators user interface

Relevant technical choices:

• removes the box-shadow, as the current style can be done with border only
• added bonus: border works also in Windows High Contrast mode
• on focus, the border style changes to dashed

Screenshot of normal state:

Screenshot of focus state:

Test instructions

• on trunk:
  • use the keyboard and focus the first separator
  • notice there's no style change
  • use arrow keys to focus the other separators: still no style change
• on this branch:
  • build the CSS
  • repeat the steps above
  • notice the style change

Update: test also in the Configuration wizard:

• yarn link the yoast-components branch 12144-title-separator-focus
• build the assets
• go in the Configuration wizard > step 9
• check the title separators look the same as in the Settings page and the focus style works the same

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Fixes #12144

[afercia]

Forgot to make the same change in the configuration wizard. On it.

[abotteram]

CR 👍

[afercia]

Forgot to update the PR description with the design feedback from Yoast/yoast-components#826 (comment)