Add FontStyle.Strikethrough support

[ghost]

This PR adds support for strikethrough fontStyle from Textmate spec and Sublime Text ecosystem.
Part of #43504 epic.

[ghost]

Does the markup.strikethrough Textmate token result in a strikethrough?

[alexdima]

I have tested that everything is wired correctly using:

"editor.semanticTokenColorCustomizations": {
    "rules": {
        "enum": {
            "fontStyle": "strikethrough"
        }
    }
},
"editor.tokenColorCustomizations": {
    "textMateRules": [
        {
            "scope": "markup.bold.markdown",
            "settings": {
                "foreground": "#FF0000",
                "fontStyle": "strikethrough"
            }
        }
    ]
},

I then tested it with a TypeScript file where there was an enum and with a markdown file where there was some bold text.

---

In order for ~~abc~~ to be rendered with a strike-through out-of-the-box in markdown, two more things need to happen:

1. the markdown grammar needs to emit a special scope like e.g. markup.strikethrough when encountering ~~...~~.
2. all builtin themes need to add a rule targeting markup.strikethrough and setting the fontStyle to strikethrough.

@sndst00m I suggest we merge this PR because it adds support for strikethrough, and then pursue 1. and 2. in a subsequent PR

cc @mjbvz @aeschli

[ghost]

I have already made the PR for strikethrough in the Markdown grammar🥳 microsoft/vscode-markdown-tm-grammar#100