Forms: Update registerField to be more defensive

[enejb]

Currently there are plugins that implement their own form Ajax form submission.
This PR tries to help them by removing js errors for them.

Proposed changes:>

• Detect if the fields exists in the context and if not create 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)?

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

No

Testing instructions:

Add a new form.
Does the form submit as expected?

[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 WoA dev 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 (Jetpack), and enable the update/be-more-defensive-in-forms branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack update/be-more-defensive-in-forms

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/modules/form/view.js	0/274 (0.00%)	0.00%	2 ❤️‍🩹

Full summary · PHP report · JS report