Enable babel-loader cache, document webpack and babel-loader cache

[askoufis]

From the changeset:

Sku's webpack configuration now configues babel-loader to emit a cache to node_modules/.cache/babel-loader. The primary use case for this cache is speeding up production builds. It can also speed up local development in situations where the webpack cache is invalidated.

The emitted cache files take up a relatively small amount of space (10s of MBs, and the compress quite well), making it much more practical to cache on CI, relative to the webpack cache.

In a moderately-sized, internal monorepo, this cache dropped the sku build time in CI from an average of 4m 11s (across 5 builds) to an average of 3m 3s (across 5 builds). The cost of each cache file is roughly an extra 6s (saving and restoring the cache) and 8.3mb of storage.

Caching node_modules/.cache/babel-loader across all of sku's fixtures decreased test time by ~20s in sku's CI.

I've added docs for both this cache and the existing webpack cache in the Extra features docs. The docs are relatively high-level, and don't go into details on actually setting up a cache in CI.

[changeset-bot]

🦋 Changeset detected

Latest commit: de199a1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
sku	Patch

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[michaeltaranto]

Just checking, is the trailing "-" correct?

[askoufis]

All these examples have a trailing hyphen, so I'd say yes.

It's probably not required, I think the restore key is just meant to be a non-unique prefix of the key, so it would probably work without the - too.