Option to define snapshot directory name

[silverwind]

Is your feature request related to a problem? Please describe.

I prefer to name my snapshot directory snapshots instead of __snapshots__. I feel the underscores are unnecessary noise.

Describe the solution you'd like

A CLI option --snapshot-dirname <name>

Describe alternatives you've considered

None

Additional context

In JS, I use vitest's resolveSnapshotPath option for the same, it is a bite more flexible as it allows full customization of the path, but I guess I would be happy with a static option just to define the directory name.

[noahnu]

It's possible to implement a custom dirname via a custom snapshot extension. I agree though that first-party support would be welcome and preferred.

We reference the snapshot name in a couple places, mostly coming from the base extension abstract class. We don't currently expose syrupy options to the extension itself though. This would be far easier after #816 is implemented. We might be able to come up with a bit more of a hacky solution or workaround until PR 816 and subsequent related changes are merged though.

[noahnu]

Very loosely related to #868 (both deal with snapshot names, though the other issue is a bug, this one is a featue request, linking for discoverability).