webview js: add Babel optional-chaining transform
webview js: add Babel optional-chaining transform

Add the Babel transform for optional chaining to the webview JS build
script:

   https://github.com/tc39/proposal-optional-chaining

(The proposal is currently at stage 3, and is already present in the
main React Native compilation preset besides.)

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile