Test: Custom data for HTML and CSS

[octref]

Testing #62976 and #64164

• Windows @dbaeumer
• anyOS @kieferrm

Complexity: 3

Instructions:

• Read https://github.com/Microsoft/vscode-docs/blob/vnext/release-notes/v1_31.md#html-and-css-custom-data-support and understand what it does.
• These two samples should illustrate the usage of the custom data: https://github.com/octref/simple-customdata and https://github.com/octref/svg-data
• Here are the more detailed doc:
  • HTML Custom Data
  • CSS Custom Data

Test the following:

• With html.experimental.customData and css.experimental.customData, you can make VS Code understand custom HTML tags/attributes and custom CSS properties/atDirectives/pseudoClasses/pseudoElements
• You are able to pack the JSON files into an extension using contributes.html.experimental.customData and contributes.css.experimental.customData.

[dbaeumer]

@octref this is blocked by #67544

[octref]

@dbaeumer Do you mind testing starting from https://github.com/octref/simple-customdata? I've added precautions against the wrong settings, but it won't be available until next insider.

The settings should look like (example):

{
  "html.experimental.customData": ["./html.json"],
  "css.experimental.customData": ["./css.json"]
}