[css] Support <angle> units icon in CSS

In Firefox 49, there is an icon in the front of the angle unit, but the angle of the icon is constant.

I hope:

• The VSCode can be a dynamic response to the actual point of view.
• <kbd>Shift</kbd> + click to change the unit format.

Spec: https://drafts.csswg.org/css-values/#angles

[aeschli]

Several layers here:

• UI: The vscode API lets you add ‘before’ decorators, that’s how you get the image or character in. You can do that in your own extension or contribute it to the CSS extension.
• Language service: To know where the angles are, you need to parse the CSS document. In the case of the CSS extension, all the language smartness is in the CSS language service, So you need to extend the css language service with a new API. Here’s how this is done for color decorators.
• Wiring up: The CSS extension runs the language support in a separate process, the ‘CSS language server’ using the LSP protocol. The CSS language server needs a new LSP request that the extension can invoke to get the location of all angles. The server just calls new css language service API you added before. Here’s an example how this is done on the CSS server side and the CSS client side

[Zzzen]

😳 I’ve forgotten it.