feat: add ambient type declarations for Netlify.env (#428)

🎉 Thanks for sending this pull request! 🎉

Please make sure the title is clear and descriptive.

If you are fixing a typo or documentation, please skip these
instructions.

Otherwise please fill in the sections below.

**Which problem is this pull request solving?**

Example: I'm always frustrated when [...]

**List other issues or pull requests related to this problem**

Example: This fixes #5012

**Describe the solution you've chosen**

Example: I've fixed this by [...]

**Describe alternatives you've considered**

Example: Another solution would be [...]

**Checklist**

Please add a `x` inside each checkbox:

- [ ] I have read the [contribution
guidelines](../blob/master/CONTRIBUTING.md).
- [ ] The status checks are successful (continuous integration). Those
can be seen below.

package.json

@@ -6,7 +6,8 @@
   "description": "JavaScript utilities for Netlify Functions",
   "files": [
     "dist/**/*.js",
-    "dist/**/*.d.ts"
+    "dist/**/*.d.ts",
+    "types/**/*.d.ts"
   ],
   "scripts": {
     "build": "tsc",

types/index.d.ts

@@ -0,0 +1,6 @@
+import type { getNetlifyGlobal } from '@netlify/serverless-functions-api'
+
+// Ambient type declarations
+declare global {
+  const Netlify: ReturnType<typeof getNetlifyGlobal>
+}