feature #111 [LiveComponent] Adding rollup to make an esm build (weaverryan)

This PR was squashed before being merged into the main branch.

Discussion
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[LiveComponent] Adding rollup to make an esm build

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| Tickets       | Addresses `Bug A` from #102
| License       | MIT

Hi!

This changes how LiveComponent is bundle. There are a few important pieces:

1) I decided to continue support IE 11. The difference in the unminified file size (including polyfills) is about 5kg (56kb vs 61kb).

2) I'm building ESM only right now. I have considered a UMD build... but I don't know if there is a real use-case. In practice, this library will always be used in a build system... but I cannot find any other libraries that only output ESM.

3) This sets `babelHelpers` to `runtime`, which means that instead of embedding the babel helpers (like `_typeof` https://github.com/symfony/ux/blob/e239983ef55805224df7eef4fe7eff089c70b7cd/src/LiveComponent/assets/dist/live_controller.js#L3) in every file, they all import from ``@babel`/runtime`.

4) We're testing the `src/` files, not the `dist/` files.

I don't have a lot of experience with compiling files for the frontend - and there seems to be a lot of variability on how to do it. If you see anything weird, let me know :).

Cheers!

Commits
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10929f4 [LiveComponent] Adding rollup to make an esm build

Author: weaverryan

src/LiveComponent/assets/babel.config.json

@@ -6,13 +6,13 @@
                 "targets": {
                     "node": "current",
                     "ie": "11"
-                },
-                "useBuiltIns": "entry",
-                "corejs": "3.14.0"
+                }
             }
         ]
     ],
+    "exclude": "node_modules/**",
     "plugins": [
-        "@babel/plugin-proposal-class-properties"
+        "@babel/plugin-proposal-class-properties",
+        "@babel/plugin-transform-runtime"
     ]
 }