build(babel): Add optional chaining proposal to babel config

[mcous]

overview

As discussed on Slack, this PR adds @babel/plugin-proposal-optional-chaining to our Babel plugin chain. This enables the following syntax:

// before
const foo = bar && bar.baz && bar.baz.qux

// after
const foo = bar?.baz?.qux

Flow is able to type this syntax correctly.

changelog

• Add optional chaining proposal to babel config

Note: we also discussed adding @babel/plugin-proposal-do-expressions, but Flow does not support this plugin at a syntax level, so we can't move forward (facebook/flow#560).

review requests

I've made one example change in source to demonstrate the kinds of benefits we could see. A couple things to repeat based on our discussions on Slack and in real life:

• This syntax is easy to overuse. We should be careful and purposeful when we use it, and add comments if anything about it's usage isn't obvious
• The optional function syntax in this proposal is pretty ugly and a little hard to mentally parse
  • The technical reasoning behind the syntax decisions seems sound, but I still think we should probably avoid using it for function calls

[codecov]

Codecov Report

Merging #3201 into edge will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             edge    #3201      +/-   ##
==========================================
+ Coverage   53.17%   53.17%   +<.01%     
==========================================
  Files         691      691              
  Lines       20403    20402       -1     
==========================================
  Hits        10849    10849              
+ Misses       9554     9553       -1

Impacted Files	Coverage Δ
babel.config.js	0% <ø> (ø)	⬆️
...src/components/AppSettings/AdvancedSettingsCard.js	0% <ø> (ø)	⬆️

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[IanLondon]

[Kadee80]

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