[WIP] Add devcontainer / Codespaces support

[helen]

Pulling over a PR I was originally noodling on here: helen#53

This works - you can spin up a new Codespace on this branch and after waiting patiently for the setup script to run (it's too quiet, you have to watch setup.log to check on progress), you should be able to go to the preview URL and log in and everything. A 2-core works fine, 4-core is quite speedy but of course uses double the core hours.

However, I don't think this is ready to go into core :) I don't think it needs to be perfect but it feels a little weird to use the Docker-published wordpress image, and there are some details in here that don't feel right either.

Here are the lingering todo items I currently have:

• Do we actually need to be using the www-data user?
• Can we use the same containers that wp env uses?
• Allow some output to the terminal so it's clear what's happening
• Sometimes navigating to /wp-admin kicks me to https://localhost/wp-admin - no idea why, going to wp-login.php works fine, maybe something in the container somewhere
• How do tests run here? (Do they run, are they slow/fast/middling, etc.)

Trac ticket: https://core.trac.wordpress.org/ticket/57187#ticket

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[helen]

Closing in favor of #4031