refactor!: Refactor snapshot grouping

[aawsome]

This PR refactors the snapshots grouping and removes the grouping-related Repository methods - Repository methods now only work on Vec<SnapshotFile>.

Use Grouped instead which allows to switch from Vec<SnapshotFile> to Grouped<SnapshotFile> and vice versa.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.4%. Comparing base (60f8ea1) to head (6daa86f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/repofile/snapshotfile/grouping.rs	60.3%	25 Missing ⚠️
crates/core/src/commands/forget.rs	0.0%	13 Missing ⚠️
crates/core/src/commands/backup.rs	50.0%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/repofile/snapshotfile.rs	65.1% <ø> (+2.2%)	⬆️
crates/core/src/repository.rs	54.8% <ø> (+0.2%)	⬆️
crates/core/tests/integration/snapshots.rs	100.0% <ø> (ø)
crates/core/src/commands/backup.rs	56.8% <50.0%> (ø)
crates/core/src/commands/forget.rs	79.1% <0.0%> (-3.7%)	⬇️
crates/core/src/repofile/snapshotfile/grouping.rs	60.3% <60.3%> (ø)

... and 25 files with indirect coverage changes

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