added example for plugin explorer

SQUASHED: AUTO-COMMIT-demos-stefan-plugin-explorer-README.md,

demos/stefan/plugin-explorer/README.md

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+# Babel Plugin Explorer Demo
+
+The babel.js plugin explorer allows developers to live program plugins to the babel transpiler used in Lively4.
+Live programming provides instant feedback on the example input.
+Further, Lively's self-sustaining nature allows you to apply changes to the compiler on-the-fly, thus, affecting all future module loads without the need to restart lively.
+
+<script>
+<button click={async evt => {
+    const explorer = await lively.openComponentInWindow('lively-plugin-explorer', undefined, lively.pt(1000, 800))
+                explorer.livelyExample();
+  }}>Start Babel Plugin Explorer</button>;
+</script>
+
+You can go to the trace visualization using the <button><i class='fa fa-search'></i></button> button in top right corner.