:pen: Add support for whitelabelling to MyST CLI

The current plan for Jupyter Book is to have it be a thin white-labelled distribution of MyST-MD. The intention here is to avoid having to maintain a separate application with its own release cadence. Instead, we'll just re-skin MyST whilst making it clear that Jupyter Book is MyST.

This PR suggests one way to do this, by reading from the environment. Using this approach will require us to be careful in how we write our error messages. I added four functions:

binaryName() - the name of the binary e.g. myst or jupyter book
readableName() the human-readable name of the application, e.g. myst or Jupyter Book (via MyST)
homeURL() - the URL of the application
baseConfigs - a list of URLs / file paths of base configurations

These each return default values unless the corresponding

MYSTMD_BINARY_NAME
MYSTMD_READABLE_NAME
MYSTMD_HOME_URL
MYSTMD_CASE_CONFIGS

are set.

N.B. we probably could have these be static and read the env var at import time ... but I don't know if that's a standard pattern.

An example using Jupyter Book:

❯ MYSTMD_HELP_URL="https://jupyterbook.org" MYSTMD_BINARY_NAME="jupyter book" MYSTMD_READABLE_NAME="Jupyter Book (myst)" npx myst init

Welcome to the Jupyter Book (myst) CLI! 🎉 🚀

jupyter book init walks you through creating a myst.yml file.

You can use Jupyter Book (myst) to:

 - create interactive websites from markdown and Jupyter Notebooks 📈
 - build & export professional PDFs and Word documents 📄

Learn more about this CLI and MyST Markdown at: https://jupyterbook.org

✅ .gitignore exists and already ignores Jupyter Book (myst) outputs
✅ Project already initialized with config file: myst.yml
✅ Site already initialized with config file: myst.yml

? Would you like to run jupyter book start now? (Y/n)

This PR also adds a whitelabelling indicator to --help:

❯ MYSTMD_HELP_URL="https://jupyterbook.org" MYSTMD_BINARY_NAME="jupyter book" MYSTMD_READABLE_NAME="Jupyter Book (myst)" npx myst --help
Usage: myst [options] [command]

Jupyter Book (myst) is powered by MyST-MD. See
https://mystmd.org for more information.

Options:
  -v, --version               Print the current version of Jupyter Book (myst)
  -d, --debug                 Log out any errors to the console
  -h, --help                  display help for command

Commands:
  init [options]              Initialize a Jupyter Book (myst) project in the current directory
  build [options] [files...]  Build PDF, LaTeX, Word and website exports from MyST files
  start [options]             Start the current project as a website
  clean [options] [files...]  Remove exports, temp files and installed templates
  templates                   List and download templates

Closes https://github.com/executablebooks/mystmd/issues/1365

executablebooks / mystmd

:pen: Add support for whitelabelling to MyST CLI #1368