Issue Type: Bug

Testing #67306.

I had the following issues with the html custom data schema when using it to drive intellisense and validation of my customData definitions:

• no intellisense for valueSet for global attributes
• no intellisense in the top level array of globalAttributes and valueSets
• no intellisense in the top level array in valueSets
• no validation warning when defining a valueSet like this

            "values": [
                "_value21",
                "_value22"
            ]

VS Code version: Code - Insiders 1.31.0-insider (9606490, 2019-01-30T15:15:08.172Z)
OS version: Darwin x64 18.0.0