Resolve remaining issues with the MDX-oriented `eslint` configuration

[novusnota]

Currently, it:

• breaks on valid math syntax, which was discovered in feat/fix: latex integration with common $...$ and $$...$$ syntax #1744
• breaks on some expressions, as seen in feat/fix: spellchecking and link checking in CI #1743. See the "Could not parse expression with acorn" in one of the earlier run logs there

Related to #1746.