wpcom-proxy-request: Return Promise if no callback specified (Take 3)

[sirreal]

Reverts #39823
Take 3 of #39683:

Changes proposed in this Pull Request

The promise wrapper introduced by @andrewserong et al. in #38823 seems generic enough for it to make sense to have it in wpcom-proxy-request. This PR moves it there. This follows precedent set by wpcom.js.

Note that this is a technically a breaking change, as noted in the package's History.md:

• [...] Return Promise (rather than XMLHttpRequest instance) if no callback argument is provided.

  • In practice, most people have probably been using the callback rather than the returned XMLHttpRequest instance, so this shouldn't be a breaking change for most.

Testing instructions

Verify that Gutenboarding still works (esp account and site creation).

Conflicting changes had been merged in the interim. I've updated the imports to fix the issue from #39722

[matticbot]

Test live: https://calypso.live/?branch=revert-39823-revert-39722-revert-39721-revert-39683-add/wpcom-proxy-request-promise-return

[matticbot]

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

App Entrypoints (~315 bytes added 📈 [gzipped])

name                   parsed_size           gzip_size
entry-main                  +235 B  (+0.0%)      +87 B  (+0.0%)
entry-login                 +235 B  (+0.0%)      +79 B  (+0.0%)
entry-jetpack-cloud         +235 B  (+0.0%)      +81 B  (+0.0%)
entry-domains-landing       +235 B  (+0.0%)      +78 B  (+0.0%)
entry-gutenboarding         +165 B  (+0.0%)      -10 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.

[sirreal]

A centralized set of generic controls for wpcom-proxy-request and generic fetch could make this testing easier.

[ockham]

Follow-up material maybe? 😬

[sirreal]

I've pushed fixes for the issues introduced by merge, the auth store work was incompatible with the other changes from this branch.

This should be ready to land now.

[ockham]

Third time's a charm! 🍀 🤞