Compat: Use core-js-url-browser for URL polyfill

[aduth]

Previously: #19871
Related: https://github.com/Financial-Times/polyfill-library/issues/462, #20172 (comment), Trac#49360

This pull request seeks to resolve issues where the current polyfill for URL introduced in #19871 is ineffective:

• The condition will never be satisfied, since window.URL does exist in IE11. It's specifically the constructor which is not supported (see related comment).
• The chosen polyfill is non-spec-conformant in a way that we need it to be (https://github.com/Financial-Times/polyfill-library/issues/462, URL: Conform to URL Living Standard definition of valid URL #19871)

The proposed changes improve the condition, and use core-js-url-polyfill in place of polyfill-library for the URL polyfill. This is a package I've published to NPM, and is a simple wrapper of core-js's URL polyfills. See Trac#49360 comment for more context on why a wrapper module is needed.

Testing Instructions:

NOTE: The following can't yet be tested, because there is an unrelated error in IE11 preventing the page from being loaded. (TBD PR)

Prerequisites:

• Use Internet Explorer
• Either run WordPress 5.3.x (or any version earlier than 5.4-beta / trunk), or comment this line in the local copy of your branch:

  • gutenberg/lib/compat.php

    Line 55 in 5a5564b

    return;

  • This is needed because Gutenberg's polyfill script is not registered if there is already one registered for wp-polyfill-url. Since this was already patched to core in 5.4-beta, the changes would not otherwise take effect.

Test:

1. Navigate to Posts > Add New
2. Open F12 Developer Tools Console
3. Observe success: new URL( 'http://example.com' );
4. Observe thrown error: new URL( 'invalid' );

[github-actions]

Size Change: 0 B

Total Size: 864 kB

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[aduth]

Per #20628 (comment) , I've rebased the branch and updated script registrations in 9fa86b9. Specifically, calls to gutenberg_register_vendor_script have been moved into the gutenberg_register_vendor_scripts function, to ensure that those scripts are downloaded as part of the plugin build script. I've confirmed that with these changes, the two vendor scripts (+minified variant) are included in the plugin ZIP file.

There is one minor difference to note: Previously, the logic would avoid calling add_inline_script if it could infer that the script was registered by core (i.e. running WordPress 5.4.0+). With the new setup, it's not as simple to detect whether the script registration occurred in core or in gutenberg_register_vendor_script, so the condition was removed. In practice, this means there may be a duplicate inline script for wp-polyfill, but it should make no effective difference, since the second condition should be satisfied by the loading of the script of the first inline script if the polyfill was needed (i.e. still at most one polyfill script loaded).

cc @mcsf

[aduth]

Note that this is meant to complement the downstream core revision r47416.