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

[ockham]

Take Two of #39683. Reverts #39721.

• TODO: Fix the issue documented in Revert "wpcom-proxy-request: Return Promise if no callback specified" #39721.
• The issue is only present in Docker. To repro, run npm run distclean && npm ci && npm run build-docker.

---

Original PR desc:

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).

[matticbot]

Test live: https://calypso.live/?branch=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]

Let's try again!