Build/Test Tools: Add a devcontainer for Codespaces support.
This adds a devcontainer configuration tested for use with GitHub Codespaces. It is currently built on top of the Docker wordpress image for initial trial purposes. We should eventually be using our own containers to fully own the environment.
This should be considered a beta run to get a sense of how well cloud dev containers work for core contributors, in particular at contributor day events with limited bandwidth and mobile devices.
Props samruddhikhandale, dinhtungdu, helen, craiglpeters, sam1el, wirecat.
See #57187.