fix: improve ESM output to support Deno, Bun and Edge runtimes

[stipsan]

Supersedes #13.
Based on new research and findings on how conditional package exports actually works in Bun, Deno, Cloudflare Workers and Edge runtimes.

Blocked by sanity-io/get-it#25

Testable by

npm i @sanity/client@esm

As well as

const {default: createClient} = await import('https://esm.sh/@sanity/client@esm')

const {default: createClient} = await import('https://cdn.skypack.dev/@sanity/client@esm')

[rexxars]

Nice work! Have not tested this in all environments, but trust that you have and the tests we have in place are sufficient :)

Still think we should release this as a major version, just in case.

[github-actions]

🎉 This PR is included in version 4.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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