Use JsonDocument instead of TextDocument

[jdesrosiers]

I figured out how to define a JsonDocument that extends TextDocument so we can add JSON-specific capabilities. The primary driver for this was that it allows the JsonDocument to handle parsing the document so each feature doesn't have to parse the document individually.

I expect we'll add more things here as we go including determining the schema that applies to the document.

[srivastava-diya]

The JsonDocument model is very helpful having JSON specific methods like findNodeAtPointer and getParseErrors directly on the document will make hover, schema store integration, and completions much cleaner. Looking forward to building on top of this.