feat(admin, admin-ui, medusa-js, medusa-react, medusa): Support Admin Extensions (#4761)

Co-authored-by: Rares Stefan <948623+StephixOne@users.noreply.github.com>
Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

packages/admin-ui/scripts/create-dev-entry.js

@@ -0,0 +1,23 @@
+const fse = require("fs-extra")
+const path = require("path")
+
+function createDevEntryFile() {
+  const devEntryContent = `
+    const extensions = []
+
+    export default extensions
+  `
+
+  const devEntryPath = path.resolve(
+    __dirname,
+    "..",
+    "ui",
+    "src",
+    "extensions",
+    "_main-entry.ts"
+  )
+
+  fse.outputFileSync(devEntryPath, devEntryContent)
+}
+
+createDevEntryFile()