Block Editor: Link Control: Initialize inputValue state from value

[aduth]

Previously: #18225 (comment), https://github.com/WordPress/gutenberg/pull/19396/files#r368127421

This pull request seeks to change the behavior of the LinkControl component to initialize its inputValue state as being derived from the value prop.

This...

• ...Avoids the need for a setMode utility function altogether
• ...Ensures better syncing of the relationship between inputValue state and the value prop
• ...Resolves inconsistency where not all mode changing would occur through setMode [1] [2]

There is also one change in behavior, inherited from #19462:

• The edit field is prepopulated with the link URL, rather than with the link title (see discussion).

Testing Instructions:

Ensure unit tests pass:

npm run test-unit

Verify there are no regressions in the behavior of link control.

Notably:

• Adding a new link should show an empty link field
• Editing an existing link should prepopulate the link field with its URL
• Editing and then re-editing while the link dialog is still shown should still reflect the updated value

[retrofox]

Tested an LGTM

• Adding a new link should show an empty link field
• Editing an existing link should prepopulate the link field with its URL
• Editing and then re-editing while the link dialog is still shown should still reflect the updated value

Simpler and clearer. 🙏 .