uppy CDN: Export UIPlugin and BasePlugin (#4774)

Export UIPlugin and BasePlugin

packages/uppy/index.mjs

@@ -1,6 +1,10 @@
 // Core
 export { default as Uppy, debugLogger } from '@uppy/core'
 
+// Plugin base classes
+export { default as UIPlugin } from '@uppy/core/lib/UIPlugin.js'
+export { default as BasePlugin } from '@uppy/core/lib/BasePlugin.js'
+
 /**
  * @deprecated Use `Uppy` instead of `Core`
  */