Describe the enhancement you'd like

It would be useful if it were possible to use spaces in labels, like so:

(my label)=
# My header

Ref for [](my label).

In executablebooks/meta#467 (comment) we discovered that, while reStructuredText supports spaces in labels, like .. _my label:, the MyST parser does not.

Benefit

This would be useful for people who are coming from the rST / Sphinx ecosystem, since this is already supported in rST. However it's probably not a huge number of people using labels this way because most seem to use - or _ for this kind of thing.

Implementation

The fix for this would probably somewhere around here: