Move Autoprefixer's config in package.json.

This is so other plugins can use it too.

build/postcss.config.js

@@ -5,27 +5,6 @@ module.exports = (ctx) => ({
     sourcesContent: true
   },
   plugins: {
-    autoprefixer: {
-      browsers: [
-        //
-        // Official browser support policy:
-        // https://getbootstrap.com/docs/4.0/getting-started/browsers-devices/#supported-browsers
-        //
-        'Chrome >= 45', // Exact version number here is kinda arbitrary
-        'Firefox ESR',
-        // Note: Edge versions in Autoprefixer & Can I Use refer to the EdgeHTML rendering engine version,
-        // NOT the Edge app version shown in Edge's "About" screen.
-        // For example, at the time of writing, Edge 20 on an up-to-date system uses EdgeHTML 12.
-        // See also https://github.com/Fyrd/caniuse/issues/1928
-        'Edge >= 12',
-        'Explorer >= 10',
-        // Out of leniency, we prefix these 1 version further back than the official policy.
-        'iOS >= 9',
-        'Safari >= 9',
-        // The following remain NOT officially supported, but we're lenient and include their prefixes to avoid severely breaking in them.
-        'Android >= 4.4',
-        'Opera >= 30'
-      ]
-    }
+    autoprefixer: {}
   }
 })

docs/4.0/getting-started/build-tools.md

@@ -36,7 +36,7 @@ Our [package.json]({{ site.repo }}/blob/v{{ site.current_version }}/package.json
 
 Bootstrap uses [Autoprefixer][autoprefixer] (included in our build process) to automatically add vendor prefixes to some CSS properties at build time. Doing so saves us time and code by allowing us to write key parts of our CSS a single time while eliminating the need for vendor mixins like those found in v3.
 
-We maintain the list of browsers supported through Autoprefixer in a separate file within our GitHub repository. See [/build/postcss.config.js]({{ site.repo }}/blob/v{{ site.current_version }}/build/postcss.config.js) for details.
+We maintain the list of browsers supported through Autoprefixer in a separate file within our GitHub repository. See [/package.json]({{ site.repo }}/blob/v{{ site.current_version }}/package.json) for details.
 
 ## Local documentation
 

package.json

@@ -121,6 +121,16 @@
     "scss/**/*.scss",
     "LICENSE"
   ],
+  "browserslist": [
+    "Chrome >= 45",
+    "Firefox ESR",
+    "Edge >= 12",
+    "Explorer >= 10",
+    "iOS >= 9",
+    "Safari >= 9",
+    "Android >= 4.4",
+    "Opera >= 30"
+  ],
   "jspm": {
     "registry": "npm",
     "main": "js/bootstrap",