Specify markup.strikethrough in themes

[CptPotato]

Currently markdown text for strike through isn't visualized in any way.

It looks like everything is already in place for this to work. Only the themes need updating, by adding the missing theme key:

"markup.strikethrough" = { modifiers = ["crossed_out"] }

The result looks like this:

This seems like a rather trivial change. I can take a look at updating the themes if you want.

[CptPotato]

Strike through in doc comments doesn't work with this change, yet, similar to bold and italic. If I understand correctly, it requires changes to this match:

helix/helix-term/src/ui/markdown.rs

Lines 196 to 197 in 4a3b776

	for event in parser {
	match event {