Add migration cli

[paul-sachs]

Created a CLI that migrates from old packages to new.

• Using jscodeshift to guarantee that we are only modifying imports and not arbitrary code
• Testing with jest since jscodeshift have a bunch of helpers to use with jest. We can migrate those to jasmine for consistency

[timostamm]

This looks very promising, now we just have to sort out the details.

Blocking on jest, we already have enough dependencies. I'm okay with replacing it later, when we know better what needs testing.

[paul-sachs]

@timostamm all great points, i had started on this tool separate from connect-es, so i'll migrate the packages/testing to something more akin to what's in connectrpc already. This was mostly just porting things over as is. I'll tune it up.

[paul-sachs]

@timostamm I've fixed the issue with parsing you had with the custom-transport.ts file and we now use different parsers for different filetypes. This does raise the risk that someone might have some custom babel setup but i've added a section to the readme to describe other ways to run our transforms.