Introduce @tus/utils to simplify building/publishing

[Murderlon]

Problems:

• In all packages we have @tus/server in peerDependencies but also in devDependencies (for tests).
• peerDependencies changes aren't noticed by package install unless it's a major. This is why changeset (our versioning and release tool) always bumps all stores to a major for any kind of release to server. See #566
• Turbo can't cache correctly when running tests. For instance, file-store depends on server but server tests depend on file-store. We can't create a circular dependency so we omit file-store as dev but then changes aren't picked up.

Solution:

• Introduce @tus/utils, have it in dependencies everywhere. All things that are shared between all packages are in here. Remove @tus/server from peerDependencies everywhere.
• No breaking changes in exports.

Now we have correct dependency graphs for turbo and we can publish more correctly and easily with changesets.

This is arguably a breaking change, but I'd argue it isn't. It only is if you update one package but not the other. However that can happen all time the time in monorepo setups like this. Whether it's a bug fix that is coordinated between packages, or a feature that needs detecting in a store, you can't expect all changes to work if you don't upgrade packages together. In Uppy, our policy is it's not a breaking change if we don't change exports and no public methods and properties a are changed.

[Acconut]

I don't understand how @tus/utils helps with the peerDependencies issue. Can you elaborate on this? Or is not addressed by this PR?

[Murderlon]

As seen in the diff, there are no peer dependencies anymore and thus no problems anymore associated with them. All packages now have @tus/utils in dependencies. Similar to how this was solved in Uppy with @uppy/utils.