Build an in-browser playground

[tomhoule]

This would be useful to document exactly what code the library generates.

The idea would be a static web page that takes a query, a schema and displays the generated Rust code. We could even generate it "on the fly" as you type the query. Code-generation is done through graphql-client-codegen compiled to wasm. This could be a challenge because of the proc-macro crate. We need to solve this for the CLI use-case too so hopefully we will have a solution soon.

One thing we could explore is integrating into GraphiQL or graphql-playground but I don't know if they are extensible/plugin friendly enough.