workflow: add size-check package

packages/size-check/README.md

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+# Size Check
+
+This package is private and is used for checking the baseline runtime size after tree-shaking (with only the bare minimal code required to render something to the screen).

packages/size-check/package.json

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+{
+  "name": "@vue/size-check",
+  "version": "3.0.0-alpha.1",
+  "private": true,
+  "buildOptions": {
+    "name": "Vue",
+    "formats": ["global"]
+  }
+}

packages/size-check/src/index.ts

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+import { render, createVNode } from '@vue/runtime-dom'
+
+// The bare minimum code required for rendering something to the screen
+render(createVNode('div'), document.getElementById('app')!)