Prettier to ^1.18.0 conflicts with Typescript 3.2.1

[cartogram]

TLDR: Updating Prettier to the latest minor version requires us to be on Typescript 3.3 or later.

When attempting to update to the latest minor version of Prettier, I encountered an issue where the formatted TypeScript output added a trailing comma to Tuples.

For example

// Before
export function useSomeHook(): [
  string,
  string
] { /*...*/}

// After 
export function useSomeHook(): [
  string,
  string, <-- Trailing comma 
] { /*...*/}

This is a problem because our version of Typescript does not support trailing commas on Tuples. It looks like they were were added in Typescript 3.3 via this PR.

Work has already been started in this PR to upgrade Typescript. When that is shipped, we can upgrade to the latest prettier.

[stale]

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