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

[haoqunjiang]

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:

import { capitalize } from '@vue/shared'

If bundled by esbuild (esbuild --bundle test.js --tree-shaking=true),
the output would be tens of lines.

(Technically, it's not esbuild's bug, because Object.freeze can be overwritten, Array.isArray could be overwritten to a getter, + can have side effects… Even though we know they are side-effects-free, it would require much more work from us to make bundlers happy)

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

Credit to @hardfist for discovering this issue.

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[haoqunjiang]

There's another potential improvement to Vue's tree-shakability:

The sideEffects field was removed from the main package because we don't want bundlers to shake the registerRuntimeCompiler(compileToFunction) call.

But we can use array-style sideEffects to mark all non-runtime-entries with side effects, and let the bundlers take runtime entries as side-effects-free:

  "sideEffects": [
    "dist/vue.cjs.js",
    "dist/vue.cjs.prod.js",
    "dist/vue.esm-browser.js",
    "dist/vue.esm-bundler.js",
    "dist/vue.global.js",
    "dist/vue.global.prod.js"
  ],

(This will shake away the initDev() call in runtime bundles. But I don't think anyone would enable tree-shaking in development, so it should be fine.)

Though, maintaining such an array in the long run seems too much burden, so I didn't add it in this PR.