DEV: move deprecation silencer to a shared package (#22668)

This babel plugin is intended to supress the deprecation warnings
from building plugins, however, discourse-plugins does not actually
consume this plugin at all. Currently this happens to work due to
how the babel worker processes are shared and the timing/ordering
of the build, but it will stop working with the embroider build.

This commit extracts the plugin the a shared package so that it
can be properly consumed by discourse-plugins as well as core.

app/assets/javascripts/discourse-plugins/index.js

@@ -90,6 +90,12 @@ function parsePluginName(pluginRbPath) {
 module.exports = {
   name: require("./package").name,
 
+  options: {
+    babel: {
+      plugins: [require.resolve("deprecation-silencer")],
+    },
+  },
+
   pluginInfos() {
     const root = path.resolve("../../../../plugins");
     const pluginDirectories = fs