Link UI briefly shows unwanted rich preview for internal URLs

[getdave]

Description

Due to the introduction of loading placeholders towards the end of #31464, internal URLs will now briefly showing a "loading" state. This is not desirable.

This PR fixes this by testing against a base URL which is used to test if the URL being previewed is an internal URL. If it is then we bail out.

This baseURL has been added to the @wordpress/editor block editor settings as we cannot apply the test directly in as this must remain WordPress agnostic. Note however, that the concept of a internal URL applies to all types of application.

Step-by-step reproduction instructions

1. Create link to internal URL (ie: a Page or Post).
2. Click on link again to see preview.
3. See brief loading UI flash up.

Expected behaviour

Should not show any rich preview loading state (or otherwise) for internal URLs.

Actual behaviour

Shows a briefly "loading" UI for rich previews even if the URL is internal.

Screenshots or screen recording (optional)

Code snippet (optional)

WordPress information

• WordPress version:
• Gutenberg version:
• Are all plugins except Gutenberg deactivated?
• Are you using a default theme (e.g. Twenty Twenty-One)?

Device information

• Device:
• Operating system:
• Browser:

[getdave]

One means to solve this - which I'm currently working on - is to make the whole concept of "rich previews" less focused on external URLs and more just a generic thing for all types of URLs.

By doing this we can abstract away some of the complexity and it will make the previews easier to extend to support internal previews in the future.