feature request: support conditional exports (package.json `exports` field)

Love the style support, but would really helpful for conditional exports to also be legible to postcss-import:

{
  ...,
  "exports": {
    ".": "./dist/index.js",
    "./*": "./dist/lib/*/index.js",
    "./styles": "./dist/styles/index.css",
    "./styles/*": "./dist/styles/*/index.css",
    "./package.json": "./package.json"
  },
  ...,
}

The main reason I suggest this is because I cannot specify an @import "my-package/styles" statement in a way that is supported by both esbuild (supports standardized package.json exports, but not nonstandard style) and postcss-import (where situation is reversed). my-package/styles is the only thing that works for esbuild, but my-package is the only thing that works for postcss-import.

I would recommend following the Node doctrine of “if exports exists, it overrides anything else” (i.e. style field). Also thank you for maintaining this project, it is a core part of modern development workflows IMO, only reason I suggest the enhancement.

[RyanZim]

The module we use for resolution doesn’t support exports yet; so that’s a technical blocker: https://github.com/browserify/resolve/issues/222.

Presumably, we’d want to support some kind of style-specific conditional export as well, so you could export both JS and CSS at ., similiar to how using both main & style works today. Does esbuild have anything like this?

[RyanZim]

Relevant: https://nodejs.org/api/packages.html#community-conditions-definitions