Forms: move Single and Multiple Choice input caret to the end on focus

[monsieur-z]

Fixes https://github.com/Automattic/vulcan/issues/412

Proposed changes:

Following #38246, after removing a field option with the delete key, the previous item gets the focus, but the caret doesn't move to the end. This PR fixes it.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

n/a

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Spin up a Jetpack test site with this branch
• Create a new post
• Add a form block
• Add the Multiple Choice field
• Add several options
• Start deleting them: when a previous item gets the focus, the caret should be at the end of the text

[github-actions]

Thank you for your PR!

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[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix/choice-field-caret branch.

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[coder-karen]

Tests well, looks good.