move babel dependencies into `devDependencies`

[danielegarciav]

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babel dependencies are only needed at build time and not needed by users of this library.

if left in "dependencies", users of the library are forced to download about 109 unused transitive dependencies.

[danielegarciav]

I see this package can be installed globally as a CLI to build itself for different environments, and converting babel dependencies into dev dependencies might or might not break this functionality. Will have to check...