fix: improve astro compatibilty

[stipsan]

When this pattern is used:

{
  ".": {
    "source": "./src/index.js",
    "require": "./dist/index.cjs",
    "node": {
      "import": "./dist/index.cjs.js"
    },
    "import": "./dist/index.js",
    "default": "./dist/index.js"
  }
}

astro throws an error as it will use the node.import condition and isn't prepared to handle a re-export of CJS:

error   "default" is not exported by "../dummy-commonjs/dist/index.js", imported by "../dummy-commonjs/node/index.mjs".
  File:
    /Users/cody/Developer/GitHub/pkg-utils/playground/dummy-commonjs/node/index.mjs:1:7
  Code:
    1: import cjs from '../dist/index.js'
              ^
    2: 
    3: export const format = cjs.format
  Stacktrace:
RollupError: "default" is not exported by "../dummy-commonjs/dist/index.js", imported by "../dummy-commonjs/node/index.mjs".

The solution is to add a node.module condition before node.import, that has the same value as import:

{
  ".": {
    "source": "./src/index.js",
    "require": "./dist/index.cjs",
    "node": {
      "module": "./dist/index.js",
      "import": "./dist/index.cjs.js"
    },
    "import": "./dist/index.js",
    "default": "./dist/index.js"
  }
}

This PR adds validation that ensures this pattern, and other pkg.exports ordering best practices, are used. And it will throw errors on common mistakes.