support limit markdown.styles active in single theme

[Binaryify]

markdown.styles only support css link array now, it don't support limit active in one theme, and it will cause active in global scope, if it can support object like

"editor.tokenColorCustomizations": {
        "[One Dark Pro]": {
            "textMateRules": []
        },
       
    }

that will better

[mjbvz]

My proposal is to add a data attribute inside webviews with the theme name. This would let you write css such as:

body[data-vscode-theme-name="One Dark Pro"] {
    background: red;
}

[Binaryify]

@mjbvz that's good enough :)