Add domain picker to FSE

[alshakero]

Depends on #43045.

Changes

The PR is the first step towards Domain Picker in FSE. Currently the domain picker:

1. Has a working search feature.
2. Perfectly styled.
3. Categories are intact.
4. All Confirm/close buttons work

What's left

Reading

1. I'm using a global (_currentSiteId) and Sites store to get the site title to initiate the domain search. Feels hacky.
2. I'm constructing the currentDomain by processing site.URL. Feels hacky too and doesn't contain all the domain info (is free, relevance, category, etc..)

Writing

When users set a domain (or a plan) we need to use wpcom.cart to store that information. There is a good example here. Something like this:

const cart: Cart = await wpcom.getCart( newSite.site_slug );
await wpcom.setCart( newSite.blogid, {
	...cart,
	products: [ ...cart.products, planProduct, domainProduct ],
} );

What's unclear to me is whether this will add two domains to the cart or the backened will remove one of them automatically when you add a new domain product. This will need some investigation.

Also, I'm not sure how to access wpcom library in wp-admin, since I think it's part of client folder.

Testing

1. In root, run yarn start to run Calypso
2. In apps/full-site-editing, run yarn dev --sync to run the app and sync it with your sandbox.
3. Create and sandbox a simple site.
4. Define a A8C_FSE_DOMAIN_PICKER_ENABLE flag and set it to true in PHP.
5. When you land in the editor page, a working domain picker button should be visible.

[matticbot]

Test live: https://calypso.live/?branch=add/domain-picker-to-fse

[matticbot]

Caution: This PR affects files in the FSE Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D44355-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing in the FSE Plugin for more info: PCYsg-ly5-p2

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~22 bytes added 📈 [gzipped])

name                 parsed_size           gzip_size
entry-gutenboarding        +18 B  (+0.0%)      +22 B  (+0.0%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[razvanpapadopol]

This PR is superseding #42834, right?

[alshakero]

This PR is superseding #42834, right?

Sadly I totally forgot about that one. Luckily, there is a little overlap at this point so I can steal Yan's work into mine :D

[simison]

Likely needs ^

[simison]

PRed #43058 since there seems to be NPM config problem.