[api-minor] Add the Babel targets-option to avoid transpiling code for unsupported browsers

Currently we simply use the Babel `preset-env` in the `legacy`-builds of the PDF.js library. This has the side-effect of transpiling the code for *very old* browsers/environments, including ones that we (since many years) no longer support which unnecessarily bloats the size of the `legacy`-builds.

For the CSS files we're only targeting *the supported browsers*, and it's thus possible to extend that to also apply to Babel.
One of the most significant changes, with this patch, is that we'll no longer polyfill `async`/`await` in the `legacy`-builds. However, this shouldn't be an issue given the browsers that we currently support in PDF.js; please refer to:
 - https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#faq-support
 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function#browser_compatibility

package.json

@@ -3,7 +3,6 @@
   "devDependencies": {
     "@babel/core": "^7.18.10",
     "@babel/plugin-transform-modules-commonjs": "^7.18.6",
-    "@babel/plugin-transform-runtime": "^7.18.10",
     "@babel/preset-env": "^7.18.10",
     "@babel/runtime": "^7.18.9",
     "@javascript-obfuscator/escodegen": "2.3.0",