Similar to spoilers, some Markdown flavors like GitHub's or Discord's
allow you to strike through text with ~~. Normally, the only way to do
this on most Matrix clients (including Element) is surrounding text with
<del> tags. As expected, no one knows how to do this.
So now NeoChat supports the ~~ syntax. We can reuse the existing spoiler
parser for this after making it generic. I have added new test cases for
this syntax. This does not affect the quick format bar yet.
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