Handle borders (e.g. for overline) in KaTeX

[gnprice]

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

"""

607 messages failed because of unsupported inline css property: border-bottom-width. 174 messages failed because of unsupported css class: overline-line. 174 messages failed because of unsupported css class: overline. 33 messages failed because of unsupported css class: underline. 33 messages failed because of unsupported css class: underline-line.

These are all related to borders, the overline and underline are for \overline and \underline respectively which are also rendered using borders in CSS (e.g. $$ \overline{A} $$, $$\underline{A} $$).

From the initial draft implementation for this, the tricky part was similar to SVGs where the (empty) span that has the borders applied, has to size it according to its siblings in a vlist. """

This was the 3rd item after all the functionality included in releases so far (https://github.com/zulip/zulip-flutter/issues/46#issuecomment-3043443472).