[CHANGE] PreTeXt in GitHub codespaces

[oscarlevin]

I have been working on making editing PreTeXt documents inside a GitHub codespace better.  Sorry for those of you who have run into problems in the last few weeks.  But I'm happy to say things should be much better now.  I hope.

If you would like to update an existing project to use the new codespace, you should be able to run `pretext update`, as long as you are on 2.16.1 (otherwise run `pretext upgrade` or `pip install pretext --upgrade` first).

If you start a new project using the template available at https://github.com/PreTeXtBook/pretext-codespace, you will be able to start up a codespace almost instantly.  It will then install pretext and some LaTeX automatically, but this shouldn't take more than a minute or two.  One change is that you will not have a project initially.  This way you can run create a project of whatever type you like (article, book, course, slideshow).  

Another change: to save on disk space and time to startup, the container now uses TinyTeX with only the latex packages we absolutely need.  The sample-article and -book work with what we have, but there might be other packages that we should install as well.  If you run into issues, post them on this issue: https://github.com/PreTeXtBook/pretext-codespace/issues/21.

Sagemath is no longer installed by default, but it should be easy to add it.  There is a new command in VS Code that will automate this.  Search the command palette for "PreTeXt: Install SageMath".

Oscar.