Syntax Highlighting for strikethrough

While the rendering of the `~~strikethrough~~` works as intended, I would also love to see some form of syntax highlighting, but I can't find any reference for the `.style` files to do it myself. I have tried setting

```
STRIKETHROUGH
foreground: bad55
```

but that isn't doing anything, `DEL` doesn't work either. So how can I style this in the editor?

Start by checking the editor's .style files and the existing syntax names, including STRIKETHROUGH and DEL. Determine whether strikethrough has a supported style key or whether syntax highlighting needs to be added; done means the styling works in the editor and the valid key is documented.

Written by the indexing model from the issue text.