ADD post-build hook

Allow to configure a hook to be ran whenever a build is made, so that
users can perform post-build tasks. This is especially useful for the
`dev` task, as a build is ran each time the filetree changes.

This is an opportunity for users to perform custom tasks that they may
need on every builds. To do so, they just have to declare a
POST_BUILD_SCRIPT environment variable containing the path to a script,
like this:

    POST_BUILD_SCRIPT=./post-build.cjs plasmo dev

cli/plasmo/src/features/manifest-factory/base.ts

@@ -224,6 +224,19 @@ export abstract class PlasmoManifest<T extends ExtensionManifest = any> {
         }
       })
     )
+
+    if (!process.env.POST_BUILD_SCRIPT)
+      return
+
+    const postBuildPath = resolve(process.env.POST_BUILD_SCRIPT)
+
+    if (!existsSync(postBuildPath)) {
+      console.error("Post-build file does not exist or is not readable:", postBuildPath)
+      return
+    }
+
+    const postBuild = require(postBuildPath)
+    postBuild()
   }
 
   async updateEnv() {