Handle SVGs for arrows, etc., in KaTeX

[gnprice]

From our TeX content survey, at #mobile-team > KaTeX survey results @ 💬, @rajveermalviya writes:

"""

624 messages failed because of hard fail: unexpected CSS class for vlist inner span: svg-align
382 messages failed because of unsupported css class: x-arrow.
374 messages failed because of unsupported css class: x-arrow-pad.
73 messages failed because of unsupported css class: sqrt.
27 messages failed because of hard fail: unsupported html node: svg

These are all related to SVGs, where the x-arrow and x-arrow-pad classes specifically corresponds to \xrightarrow (e.g. $$ \xrightarrow{A} $$).

From the initial draft implementation, the tricky part was to correctly size the svgs or in some cases parts of the svgs.
"""

This was the 2nd item after all the functionality included in releases so far (#46 (comment)).