Migrate existing blocks to experimental form framework

[carlomanf]

What problem does this address?

Core blocks like login form, comment form and search form offer very little customisability. For example, changing the button text from "Post Comment" to "Add Comment" or disabling the website field is impossible without using filters.

What is your proposed solution?

Re-fashion the comment form, login form and search form blocks as variations of the experimental form block. For example, removing the website field from the comment form could be as simple as deleting the relevant input block from the form, and changing the button text could be as simple as clicking and typing.

The experimental form suite still has a way to go before being viable for such a purpose. Most notably, the ability to transform between form variations via the inspector is of questionable value. If a user is customising their comment form, they would just be confused to see an option to transform it to a privacy request form. The embed block does not let you transform between variations, so why should the form block let you?

Similarly, being able to change the submissions method is likely to cause too much confusion and unintended destruction. It does not make any sense that a user should be able to submit their comment form or especially their login form by GET method. The form variations should be "siloed" such that once a variation is selected, the technical specifications are locked in and the form can only be used for the originally selected purpose.

There are other minor bugs with the experimental form suite that need to be addressed before it can be moved out of experimental status. Another one I noticed is that the form block is not able to be selected if inside a group block.

Unfortunately there has been very little attention on the experimental form suite since its initial merge and even more so since the original author retired from contributing to this repository. History has shown that stale features start getting more attention once they become dependencies for something else, which is why I think it is time for pre-existing core features like login form, comment form and search form to be migrated to this framework.

[carlomanf]

Not sure of the logistics of this, but it would be ideal if these blocks could be migrated within the forms experiment only and left unchanged outside of it.