feat(codegen): add a default prepack script

[adamthom-amzn]

Description of changes:
Doing a clean build in prepack works for both package managers and makes a
simple process of codegen && (npm|yarn) publish work out of the box. Also
adds an "omitPrepack" configuration option to allow the SDK to disable this
script for backwards compatibility.

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[gosar]

If the SDK doesn't need it, I think we should not provide this option.

[AllanZhengYP]

The change makes sense, but I'm not a big fan of the name prepack. We don't have a pack script, so the pre hook doesn't make a lot of sense. pack is a overloaded verb in JS ecosystem. What does it mean?

[adamthom-amzn]

It's automatically run as a part of npm publish and it's recommended by yarn as a place to do transpilation before pack. So you don't have to define pack at all - it's a built-in part of the lifecycle of both package managers.

[AllanZhengYP]

Since we already defined a files in the package.json, which is an allowlist of all the paths to be pulished, do we still need to call the clean in the script?

[adamthom-amzn]

Since we already defined a files in the package.json, which is an allowlist of all the paths to be pulished, do we still need to call the clean in the script?

Wanted to make sure that npm publish didn't pick up a JavaScript file in dist for a TypeScript file that had been removed from the input.

[AllanZhengYP]

Just realize TSC doesn't remove the JS files from previous builds unless with --clean: microsoft/TypeScript#30602 (comment)

Ship it!