Enable generic Embed block in dev mode

[ceyhun]

Fixes #3272

gutenberg PR: WordPress/gutenberg#30456

To test:

In dev mode:

• Check if block inserter list has only the generic Embed block (and not the ones like Youtube, Twitter, etc.)
• Try adding a generic Embed block via the inserter
• Check if Embed block is rendering in initial html in the demo app without crashing

In release mode:
Can be tested using:
WPiOS PR: wordpress-mobile/WordPress-iOS#16214
WPAndroid PR: wordpress-mobile/WordPress-Android#14388

• The block inserter shouldn't show Embed block
• If a post already has an Embed block, it should be shown as unsupported

On web:

• Check if Embed block is still working without any errors (can be rendered, added, modified)

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

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[jd-alexander]

Approved via WordPress/gutenberg#30456 (review)