chore(@vue/shared): add "sideEffects": false

As far as I can tell, all the code in this package is side-effects-free.

Without the `sideEffects` flag, bundlers rely on code analysis and
the `/*#__PURE__*/` comments for tree-shaking.
As bundlers' implementations vary, the analysic may or may not be
exhaustive.

For example, a file that contains only the following line of code:
```js
import { capitalize } from '@vue/shared'
```
If bundled by esbuild (`esbuild --bundle test.js --tree-shaking=true`),
the output would be tens of lines.

After this PR, the output will be an empty IIFE.

Credit to @hardfist for discovering this issue.

Author: haoqunjiang

packages/shared/package.json

@@ -9,6 +9,7 @@
     "index.js",
     "dist"
   ],
+  "sideEffects": false,
   "buildOptions": {
     "formats": [
       "esm-bundler",