scss grammar lack of syntax highlight of :host-context() and other experimental pseudo-classes

[alexr00]

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Description

:host-context() and other experimental pseudo-classes are not tokenized correctly.

Steps to Reproduce

1. Use the following snippet:

:host-context(.template_025) .custom_07 h2 span {
  font-weight: bold;
}

Expected behavior:
Actual behavior:

Actual on the left, expected on the right. :host-context only gets scoped as source.css.scss but I expect it to get a more specific scope.
https://user-images.githubusercontent.com/7685505/122878212-0fccb480-d36a-11eb-957c-fd1486917026.png

Reproduces how often:

100%

Versions

From commit f52ab12

Originally from @nickbanana in microsoft/vscode#126886