Mark TypeScript as an optional peer dependency for react-scripts

[dstaley]

This PR adds TypeScript as an optional peer dependency for react-scripts. This modification is necessary to get react-scripts working with Yarn v2, and will enable the TypeScript template for create-react-app to work with Yarn v2.

This PR uses the peerDependenciesMeta key, which signifies to Yarn that typescript is an optional dependency. npm will support the key in the future, in which case "typescript": "*" can be added to peerDependencies. Until that change is made, simply having peerDependenciesMeta will be enough for Yarn, and won't adversely impact users of npm.

Other relevant issues: yarnpkg/berry/pull/475

/cc @iansu

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[iansu]

@dstaley Awesome! I suppose we'll have to publish this change to get the tests passing in Yarn v2? I can merge this right away but it might not be published for a week or two as we're currently working on the next release.

[dstaley]

@iansu yeah, we'd need npx create-react-app to result in the new version being installed for the tests. It's okay if it waits for the next release! I can setup a reminder to check every couple of days to see if the new release has been published.

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[ianschmitz]

Thanks @dstaley!