Add variable panel using tree API

[joyceerhl]

Support passing variables into queries

[acao]

If you're looking to generate JSONSchema for json/jsonc/json5/hjson/yaml/etc variable completion/diagnostics/hover, I've written a utility for this! We're preparing to release support for this in monaco-graphql as well

https://github.com/graphql/graphiql/pull/1997/files#diff-ba9619999700d1a42e31cd444443419ddb4a7919bb0c625a10b3e04a5a488a44

Was just looking into how to use CustomTextEditorProvider, realising vscode-graphql should have adopted the notebook API last year, and then I found this lovely project!