Come up with a way to extract pattern-override code from the block-editor package

[kevin940726]

What problem does this address?

Originally reported in #60234 (comment).

Pattern Overrides is a WordPress-only feature. It would be nice to extract the logic and code from the block-editor package to maybe the editor package instead.

What is your proposed solution?

I haven't figured out an elegant way yet but I have some ideas.

Since that we probably shouldn't have multiple menu items with the same label, maybe we can use the label as a key to override the existing ones. For instance, the block-editor package would implement the Rename menu item normally without the overrides warning, and we'll give it a rename key.

When we try to add a new menu item with the same rename key in the editor package, then it would simply override the existing ones. This will allow us to implement the warning message there, without leaking WordPress-specific logic to the block-editor package.

[kevin940726]

Another potential improvement is the pattern overrides toolbar indicator reported here: #62317 (comment)

[talldan]

Superceded by #65486 (it proposes doing the opposite, and considering pattern overrides as a block editor first feature).