Add a widget for entity-reference fields

Assigning to myself for review...

This works great functionally! I'm just seeing a series of errors when I run tests locally. The first, as shown in the previous major test pipeline in https://git.drupalcode.org/project/linkit/-/jobs/3267712, fails because the form element description is not visible:

1) Drupal\Tests\linkit\FunctionalJavascript\LinkEntityReferenceFieldTest::testLinkEntityReferenceWidget
Behat\Mink\Exception\ElementHtmlException: The string "Start typing to find content or paste a URL and click on the suggestion below." was not found in the HTML of the element matching css "#edit-field-test-er-wrapper".

I'm not sure that the description text "Start typing to find content or paste a URL and click on the suggestion below." is quite right for a generic entity reference field, and I think we could just leave the form element description blank and have the site builder supply appropriate help text. But even with removing that, the tests fail on subsequent checks, such as the following:

1) Drupal\Tests\linkit\FunctionalJavascript\LinkEntityReferenceFieldTest::testLinkEntityReferenceWidget
Behat\Mink\Exception\ElementNotFoundException: Element matching css "li.linkit-result-line .linkit-result-line--description" not found.

I replayed this code change on the 7.x branch, which provides Drupal 11 compatibility. Ideally, we merge to that (but can also merge to 6.1.x). Will need to return to this to understand why the tests are failing as described above. Setting to "Needs work."

Thanks mate, I'll work on those fixes over the coming days/week

Wow, this would be *really* amazing, I was just googling for solutions and came across https://www.drupal.org/project/linkit_references → (but that's for D7), so having this in LinkIt core would be so great!