Add @snowpack/plugin-react-refresh to React scripts

[stramel]

Changes

Adds the @snowpack/plugin-react-refresh plugin back into the React scripts by default. The reasoning behind this is that:

• Next.js offers it by default (https://nextjs.org/docs/basic-features/fast-refresh)
• CRA offers it by default (Add Fast Refresh warning when using React < 16.10 facebook/create-react-app#9350)

It is also, pretty easy for a user to remove this plugin if they don't want to use it.

Added a peer dependency of 16.9.0 to the @snowpack/plugin-react-refresh since it requires at least that to run. (facebook/react#16604 (comment))

Testing

Tested in the @snowpack/app-template-react and @snowpack/app-template-react-typescript to ensure it was working.

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[stramel]

Although, in the CRA codebase, they say 16.10 is required. I'm going to update it to that.

[drwpow]

LGTM! I also like the idea of dogfooding this—we test out the React templates a lot so this would help us catch bugs in the react-refresh plugin quicker.

[FredKSchott]

@drwpow can you make sure that you test manually before our 2.8 release?