Add support for strikethrough formatting on text.

[smmalis37]

Made by basically just copying underline. Just like underline this does not have any support for formatting the strikethrough itself, although the underlying apis support it. I do not have access to a mac to test the coregraphics implementation (nor have I ever written mac code before), however the d2d implementation appears to work on my Win 10 box. This will need a snapshots update.

[smmalis37]

Whoops, I thought one of the files I was editing was piets, its actually core-graphics (vscode just lets you edit source of downloaded crates, fun). So that implementation won't work at the moment. Investigating what's needed.

[smmalis37]

Blocked on figuring out what to do with coregraphics. Turns out it doesn't seem to support strikethrough, so we might have to do it manually.

[smmalis37]

From Zulip:

Raph Levien: The best path forward might be to stub out the Apple implementation and open an issue to add it.

I'm fine with that if the maintainers are.

[cmyr]

Okay, I'm happy to have this go in as-is; at some point I'd like to think more about a better API for "text decorations" but this is totally reasonable for now.

We'll need an issue that this needs implementation on macOS and cairo.