vscode: support 'custom data' for HTML / CSS

Bug Description:

The extensions that offer html customData aren't showing popup insight for those html tags.

Steps to Reproduce:

1. Build the vscode extension for custom-data-sample from their vscode examples. (May need to update or remove publisher from package.json for this.)
2. Install vsix.
3. Restart workspace so that extension is reloaded (the customData extensions require reloads after installation so that the pacakge.json customdata stuff is read and used by the html language extensions)
4. Create an html file
5. Type <my-
   You should see <my-button offered in insight, but you do not.

I can offer the above vsix file for testing, or you can try some other files that do this.

Additional Information

• https://code.visualstudio.com/api/extension-guides/custom-data-extension
• https://github.com/microsoft/vscode-custom-data