Adopt monaco.MarkerTag API

[spahnke]

Use the marker tag monaco.MarkerTag.Unnecessary if the TypeScript service reports code as unused/unreachable when creating diagnostics. This way the corresponding code is rendered with a lower opacity.

Note: Currently this only works if the editor options are updated after editor creation (seems like any option will do); e.g. editor.updateOptions({ formatOnType: true });. Otherwise the opacity change is not applied. (See microsoft/monaco-editor#1619) I would consider this as blocked until the issue gets resolved because the Hint level diagnostic is not shown anymore when using the tag, even if showUnused is disabled completely.

Example code

{
    const foo = 1; // foo should be rendered with lower opacity
}

[alexdima]

Thank you! ❤️