Add a type=submit to the search suggestion buttons

[talldan]

Description

Recently pressing enter to submit a url as a navigation link in the nav block stopped working. It looks like the LinkControlSearchItem previously relied on being a type="submit" button. It was recently changed to use the Button component, which has a default type="button". This PR changes it back to a submit button to restore the functionality.

How has this been tested?

e2e tests are being worked on in #18929, which is how this was discovered.

1. Add a Navigation Block.
2. Select 'Create empty'.
3. Type a URL (e.g. 'https://wordpress.org') and press enter
4. Expect that the URL is submitted.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR. .

[draganescu]

Works perfectly!

[aduth]

When I click on a search result in the navigation block search suggestions, I see a console warning:

Form submission canceled because the form is not connected

I have also observed that this can cause end-to-end test failures:

FAIL packages/e2e-tests/specs/editor/blocks/navigation.test.js (5.528s)
  Adds Navigation links
    ✕ Should add a link with one click (4723ms)
  ● Adds Navigation links › Should add a link with one click
    expect(jest.fn()).not.toHaveWarned(expected)
    Expected mock function not to be called but it was called with:
    ["Form submission canceled because the form is not connected"]
      at Object.expect (../jest-console/build/@wordpress/jest-console/src/index.js:34:4)
          at runMicrotasks (<anonymous>)

I expect that since this button is also assigned an onClick handler, it may be conflicting with the behavior of onSubmit. To me, it would seem that the behavior of onClick should be handled in the form onSubmit handler or, conversely, there should be no onSubmit handler and we rely on onClick exclusively. In its current state, the two are prone to conflict.

[talldan]

@aduth Yep, I noticed that too and this bug when working on #18929. I've made a separate issue for it (#19056) and plan to pick it up once I work on a few of my PRs.

The root cause of this is a bit further in the past—previously these suggestions were using button elements that had an implicit type="submit". I think what's happening is that the form becomes unmounted by the onClick handler which causes the 'not connected' warning.

A fix could involve suggestions being handled by URLInput a bit like they are for the other style of suggestions:
https://github.com/WordPress/gutenberg/blob/master/packages/block-editor/src/components/url-input/index.js#L143-L224