Reusable block: Add convert to regular blocks button in the unsupported block message

[fluiddot]

Description

When editing a reusable block, the block only displays the message Reusable blocks aren't editable on WordPress for <Platform>, however, we provide an alternate way to edit it only for the current post/page via converting the block into regular blocks. As an improvement, we could add the convert button in the bottom sheet to let the user convert it easily.

Additionally, it would be nice if we update the message to make it clearer, it could be something like:
Editing reusable blocks is not yet supported on WordPress for <Platform>

Step-by-step reproduction instructions

1. Add a reusable block to a post/page.
2. Tap on the block to edit it.
3. Observe the unsupported block message.

Expected behaviour

Provide an alternative for editing the block when displaying the unsupported block message.

Actual behaviour

The block only displays the unsupported block message.

Screenshots or screen recording (optional)

WordPress information

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

Device information

• Device: iPhone 8 (simulator)
• Operating system: iOS 14.2
• WordPress app version: develop