Please update TypeScript in template to v4

[ngocdaothanh]

create-snowpack-app is still using TypeScript 3:
https://github.com/pikapkg/snowpack/blob/master/create-snowpack-app/app-template-react-typescript/package.json

Not sure if it's because it has the same problem of create-react-app:
facebook/create-react-app#9515

[FredKSchott]

Should be nothing blocking you from updating your own project to TS 4.0, unlike CRA we aren't blocked by anything on our end.

Created a PR #1090