Realign features of YAML Language Server with JSON Language Server

[JPinkney]

Currently, the YAML language server is quite a bit behind the JSON language server in terms of features and will take quite a bit of work to get everything back up to date. This issue will keep track of everything that needs to be done.

• Extend the JSON parser from vscode-json-languageservice rather then use the current version in yaml-language-server
• Replace yamlParser code that recursively builds the AST with new types
• Replace documentSymbols with vscode-json-languageservice's implementation of document symbols and change it to work with YAML's multiple documents
• Replace yamlHover with vscode-json-languageservice's implementation for json, should work the same since theres nothing specific to YAML in this.
• Replace yamlValidation with vscode-json-languageservice's implementation for json, should work the same since theres nothing specific to YAML in this.
• See if there is a way to extend the jsonSchemaService
• See what other features we can use from vscode-json-languageservice that would be relevant

By doing this we solve:
redhat-developer/vscode-yaml#154
redhat-developer/vscode-yaml#151
#133
#105

[marosg42]

@JPinkney I only wanted to bump this issue. I just started to use yaml extension in vscode and realized jsonschema draft-7 enhancement if-then-else is not available here. It would be really cool to have this available.