Add ESM build output for graphql-ruby-client

[mattander]

Summary

This adds an ES module build for graphql-ruby-client while keeping the existing CommonJS build intact for current consumers.

The package now publishes:

• existing CommonJS output through main / require
• generated ESM output under esm/ through module / conditional import exports
• subpath exports for sync and subscriptions modules, including .js subpaths for existing direct import users

The ESM build runs from prepack, so npm pack, npm publish, and Git dependency installs generate the ESM output before packaging. The ESM preparation step also normalizes generated import specifiers that Node's ESM resolver cannot load as-is, such as extensionless relative imports and directory-style package subpaths.

Why

Modern bundlers and ESM-only app setups can otherwise end up consuming the CommonJS build or needing local aliases/workarounds for imports such as graphql-ruby-client/subscriptions/ActionCableLink.

Verification

• npm test
• npm pack --dry-run
• node --input-type=module -e 'import { sync, ActionCableLink } from "graphql-ruby-client"; console.log(typeof sync, typeof ActionCableLink)'
• node --input-type=module -e 'import ActionCableLink from "graphql-ruby-client/subscriptions/ActionCableLink.js"; console.log(typeof ActionCableLink)'
• node -e 'const ActionCableLink = require("graphql-ruby-client/subscriptions/ActionCableLink.js"); console.log(typeof ActionCableLink.default || typeof ActionCableLink)'
• node -e 'require.resolve("graphql-ruby-client/cli.js"); require.resolve("graphql-ruby-client/readme.md"); require.resolve("graphql-ruby-client/CHANGELOG.md"); console.log("ok")'

[rmosolgo]

Hey, thanks for contributing this improvement! I have one question about the changes below.

IIUC, I won't need to do anything extra to include these new esm files in the next release. Does that sound right to you?

One more thought: it sounds like you validated the module structure with some local scripts. Is there a way we could work this into CI? Maybe write a spec that runs those same commands?

[rmosolgo]

Why should these be included in the exports?