feat: add ESM build

[znck]

closes #192

[Evalir]

Hi! I'd love to pick this up so we can get ESM going 😄 . Is there anything I should be aware of regarding this?

[jasonkuhrt]

@Evalir just the comments you see here

[Evalir]

Alright! I'll pick this up in the following days 😃

[benwoodward]

I'm trying to get graphql-request working with sveltekit/vite and struggling. Installed from znck's branch (had to downgrade to node 12 for this to not throw node-gyp related errors): npm i -D znck/graphql-request#master, however, the graphql-request directory in node_modules only contains an empty package.json, a README.md and a license.

What do I need to do to fix this, other than fix these conflicts ↑?

EDIT: I guess the reason the directory is empty is because yarn build only gets run via the prepublishOnly script command (which is only triggered via npm publish), and installing directly from github means that I end up with a copy of the package that hasn't been built.

EDIT: Adding a prepare script command seems to fix the project not building on install issue, https://github.com/benwoodward/graphql-request/commit/a8bf1ca823b9449844574883a408aed69ec4d1fe (when installing locally or from Github). If you're trying to get graphql-request working with Vite/Sveltekit you can install my fork using npm install -D benwoodward/graphql-request#fix/esm-build until this PR is resolved.