Add a core-version command.

[andrewbelcher]

The Issue

• Add a command for switching core version #132

Streamline to process of switching branch by adding a command.

How This PR Solves The Issue

Adds the command.

Manual Testing Instructions

ddev add-on get https://github.com/<user>/<repo>/tarball/<branch>
ddev restart
ddev core-version ^10.5

Automated Testing Overview

No tests added yet.

Release/Deployment Notes

Addition of a new core-version command to update the version configuration and code via a single command, e.g.:

ddev core-version ^11.2

[weitzman]

I think we should change our documentation from recommending a config.local.yaml file to a new .env.contrib file. Those get read by DDEV during startup. Best of all, we can use the https://ddev.readthedocs.io/en/stable/users/usage/commands/#dotenv-set command to set the value there. This way we dont have to use regex to change values.

[weitzman]

This will need a test