Adjust browserslist to compile optional chaining

[devongovett]

#2857

Two issues:

1. Browserslist was not picked up by Parcel because it was not in the package.json of each package. The root package.json was not used. Instead, we can move the browserslist to a .browserslistrc file in the root so that we don't need to repeat it. This fixes private class fields being compiled.
2. We need to lower our browser targets to a browser version that doesn't support optional chaining and nullish coalescing because webpack 4 is still widely used and doesn't support them. At some point in the future we will raise this again, so we encourage all users to upgrade webpack soon if possible.

[adobe-bot]

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[LFDanLu]

Only chrome needed a drop in supported version?

[devongovett]

Yeah just need to drop at least one down so this doesn't match: https://caniuse.com/?search=nullish%20coal