Support for chemical equations using mhchem for Katex

The next version of Joplin will feature support for chemical equations using mhchem for Katex.

For example this mhchem syntax will be rendered as below in Joplin:

\\ce{CO2 + C -> 2 CO}

C_p\[\\ce{H2O(l)}\] = \\pu{75.3 J // mol K}

\\ce{Hg^2+ ->\[I-\] HgI2 ->\[I-\] \[Hg^{II}I4\]^2-}