[graphiql] CDN usage & examples should move from umd to esm

[acao]

es6-module support is now available in safari and most of the browsers our potential users would be using

https://caniuse.com/es6-module

For graphql@4 or graphql@5, we should maintain a deprecated legacy support for UMD, but switch our documentation, examples, and primary bundle to use es6 modules natively in the browser

two options:

1. we decide to "progressively enhance" and bundle graphiql.min.js and graphiql.js as es6 compatible modules, and add graphiql.umd(.min).js bundles for those who still need UMD
2. the more cautious approach would be the opposite of the above - continue publishing the canonical bundles with UMD, and add a graphiql.esm.min.js