Improve custom data schemas

[aeschli]

html:

• add descriptions to all properties so we get nice hovers
• version can be const: 1.
• properties.name should be required
• can there be both values and valueSet?
• unnecessary attributes.anyOf, globalAttributes.anyOf and valueSets.anyOf
• globalAttributes.required and itemSet.required need to be next to the properties
• add default values and defaultSnippets for better code completion experience (as requested in Issues with html customData schema #67609)

css:

• version can be const: 1.
• simplify oneOf:[{enum with `"enum": ["standard", "experimental",...]``
• add pattern and patternErrorMessage to verify browsers
• use $ref to have avoid multiple definitions for definitions for browsers and status

[octref]

enum doesn't show suggestion for me, that's why I changed to oneOf. Others make sense, I'll update them.

[aeschli]

enum shows suggestions, we use it everywhere, there must have been some other issue.

[octref]

anyOf on Attributes: After removing it, we don't get any suggestion for the array:

[octref]

I see, that's when I should use defaultSnippets...

[octref]

Verifier:

• For setup, see https://github.com/Microsoft/vscode-html-languageservice/blob/master/docs/customData.md
• Verify version: 2 show error
• Verify that you get completions for each array, such as tags, globalAttributes, valueSets, tags.attributes in HTML custom data JSON files
• Verify that you get completions for each array, such as properties, atDirectives` in CSS custom data JSON files

[octref]

can there be both values and valueSet?

Yes. Attribute value completion gets values from both.