Enable plugins with a single configuration value

[choldgraf]

I quite like the pattern that markdown-it-py has where you basically pass a list of plugins that you wish to enable in order to extend the syntax. I feel like this would be a nice way to more gracefully extend base MyST in Sphinx as new syntax is supported.

So I'd propose that we use the following syntax to configure the extra syntax that MyST uses in Sphinx:

myst_extra_syntax = ["admonition", "deflist", ...]

instead of having a different config for each one:

myst_admonition_enable = True
myst_deflist_enable = True
...

This would also make it easier for Jupyter Book to pass through configuration to MyST (it's not difficult to do already, but would be a nice simplification):

myst:
  extra_syntax:
    - deflist
    - admonition