UIP-1639 Update formatting guidelines WRT dartfmt and trailing commas

[greglittlefield-wf]

Our formatting guidelines currently recommend against using dartfmt/dart_style on over_react code.

We recently discovered that dartfmt is capable of producing significantly more readable over_react code, due to changes in dart_style that treat Flutter code better when nested components use trailing commas.

For more info, see dart-lang/dart_style#549. Thanks to @skybrian for getting the conversation started, and to @matanlurey for suggesting we revisit the issue!

We should upgrade the formatting guidelines to:

• recommend trailing commas whenever supplying variadic children to a builder, even if it's just one
• indicate that dartfmt output, while not perfect, is acceptable when trailing commas are used

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FYI: @greglittlefield-wf @aaronlademann-wf @jacehensley-wf @clairesarsam-wf @joelleibow-wf

[rmconsole-wf]

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