myst_substitutions don't allow for dashed names

[dylanh724]

What version of myst-parser are you using?

4.0.0

What version dependencies are you using?

docutils==4.0.0
sphinx==8.1.3

What operating system are you using?

Windows

Describe the Bug

When declaring a substitution with a key that contains a dash, like foo-bar, building results in a warning:

WARNING: Substitution error:UndefinedError: 'foo' is undefined [myst.substitution]

Notice how it doesn't even detect the -bar part of the sub. Renaming to foo_bar works around this bug.

...However, we shouldn't have to (worked fine in rst_prolog when we used rst).

Expected Behavior

I should be able to declare foo-bar myst_substitution key without issues (with a dash).

To Reproduce

In conf.py:

myst_substitutions = {
    "foo-bar": "Foobar",
}