Gutenberg: Add a premium block category

[scruffian]

Changes proposed in this Pull Request:

• This adds a new "Premium" category to the block picker:

This makes it clear that these blocks are paid blocks, not free ones.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• This is part of Page Editor Plus: paAmJe-ll-p2

Testing instructions:

• Go to the post editor for a site with no plan
• Check that you don't see the Premium section
• Go to the post editor for a site with a plan
• Check that the Simple Payments button appears in a new category in the block picker:

Proposed changelog entry for your changes:

• no changelog needed

[jetpackbot]

	Warnings
⚠️	"Testing instructions" are missing for this PR. Please add some
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

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[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello scruffian! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D28489-code before merging this PR. Thank you!

[kraftbj]

Adding a request for design review since it impacts design and asking for a product review since there's not a link to a discussion about it, so not sure if product is looped in.

[eeeeevon13]

I think from a usability standpoint it might be helpful to name it "Jetpack Premium" instead of just "Premium" the icon alone doesn't follow the pattern above "Jetpack [icon]". https://imgur.com/a/whD0uU2
Aside from that the test did work for me.

[scruffian]

We're not going to change anything in the picker as it's too intertwined with other issues.