Avoid parsing documents multiple times

[jdesrosiers]

Resolves #30

Parsing TextDocuments into JsoncInstance instances is necessary for most language server features. The more features we add, the more times we will be parsing documents. Centralizing this functionality allows us to cache the result when possible, but also allows us to reduce some code duplication.

From now on, all code should use the getSchemaResources function instead of calling JsoncInstance.fromTextDocument.