[WIP] Implement new "standalone" snapshots.

[seansfkelley]

Summary

#6383 (comment)

(More detail coming later.)

Test plan

TODO

• How to name the standalone directory? Currently the PR just tacks .standalone onto the end.
  • Originally I wanted the standalone name to be the snapshot name minus .snap, but this meshes awkwardly with the resolveSnapshotPath API.
• Existing snapshots are versioned. An explicit goal of standalone snapshots is that they don't have any of the Jest export/comment metadata, since they should be legal files in whatever language they're serialized as. Should these be versioned?
• It is an explicit goal that users can provide an arbitrary extension that comes last on the standalone snapshot's file name so that other tools can infer the proper file type based on the file name. How does Jest know which files in the standalone directory are snapshots, and which are not?
  • Currently, the heuristic just looks for .snap in the filename. This seems too lax.
  • Even if users don't intend to put other files in these directories, some of their actions can cause files to appear there (.DS_Store comes to mind on macOS).
• Bikeshed: I renamed the old snapshots (internally) to "grouped" and the new ones are called "standalone".

[linux-foundation-easycla]

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