Use Git Large File Storage for snapshot images

[calda]

This PR adopts Git Large File Storage for our snapshot images. This should reduce the default size of the repo by ~130MB (the size of Tests/__Snapshots__) and will prevent the git history from growing arbitrarily large.

[calda]

After merging this PR I'll rewrite the git history of master to use LFS, so a typical checkout done by a package manager will no longer need to download any of these images or their existing git history.

To prevent breaking any existing projects referencing specific commits on master (e.g. either though using explicit commit shas in their Package.swift / Podfile / Cartfile, or as recorded in Package.resolved / Podfile.lock / Cartfile.resolved), I'll publish a tag of the current master branch. This makes sure that existing commits can still be downloaded even if they're no longer present on master. I confirmed that all three package managers support this type of change.

I decided to not rewrite the history of master. Swift Package Manager always downloads the full repository, so moving commits from master to a different branch doesn't reduce the amount of data that SPM downloads. We can't delete existing commits since that breaks existing integrations for our consumers. Fortunately adopting LFS reduces the size of a default checkout from 375MB to 215MB (which is good progress) and prevents the repo from growing significantly larger.

[calda]

ptal @erichoracek

[erichoracek]

Thanks for working on this, glad we can take this approach

[erichoracek]

@calda One thought—do we need a guide for contributors to pull later changes after force pushing to master?