fix(wax): replace unsupported astral unicode to avoid crash

This is a temporary, non-ideal fix for #693 (closed), to prevent Wax crashing. We go through the entire document (not just within MathML blocks!) and replace any of the unsupported characters from the Mathematical Alphanumeric Symbols block with similar supported characters. We should remove this fix once we have a fix in Wax.

Note that we're not replacing other astral unicode such as emojis, but it's unlikely that we'll get emojis included in a math expression.