Automatically open text notebooks as notebooks

[ctcjab]

Currently the default viewer that is used when you double click a .py (or .md, etc.) file is an all-or-nothing setting: Users can customize Document Manager settings to make e.g. all .py files open as Jupytext notebooks rather than in the text editor, so they don't have to do the right-click -> Open With -> Jupytext Notebook dance every time they want to open a .py notebook as a notebook, but then they'll have the opposite problem when they want to open e.g. helper_lib.py in the text editor.

Ideally the default viewer used to open a file could automatically be Jupytext Notebook when the telltale Jupytext metadata is found in the file.

[itcarroll]

Even with those configurables, I can't figure out a way to achieve all three of:

1. Use the md:myst format (I prefer the .md extension mostly b/c GitHub doesn't render .myst, TBH)
2. Double click to open md:myst format with Jupytext Notebook
3. Double click to open other .md files with Editor

Would love to have all three.

It's also weird to me that the file's icon does not match it's default viewer.