feat: skip btrfs snapshots by default, scan them as their own scope on enter

[5unekku]

only can test on btrfs, so unable to personally implement on other filesystems

[codecov]

Codecov Report

❌ Patch coverage is 71.70868% with 101 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.78%. Comparing base (387cf50) to head (6e9729b).

Files with missing lines	Patch %	Lines
pkg/device/extents_linux.go	0.00%	36 Missing ⚠️
pkg/analyze/subvolume.go	74.75%	26 Missing ⚠️
pkg/device/dev_linux.go	27.27%	8 Missing ⚠️
tui/snapshot.go	83.67%	8 Missing ⚠️
webui/snapshot.go	73.33%	8 Missing ⚠️
pkg/analyze/sqlite.go	72.72%	3 Missing ⚠️
tui/tui.go	66.66%	3 Missing ⚠️
pkg/device/extents.go	94.87%	2 Missing ⚠️
cmd/gdu/app/app.go	90.90%	1 Missing ⚠️
internal/common/ui.go	0.00%	1 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #689      +/-   ##
==========================================
- Coverage   90.73%   89.78%   -0.96%     
==========================================
  Files          76       81       +5     
  Lines        6843     7195     +352     
==========================================
+ Hits         6209     6460     +251     
- Misses        633      734     +101     
  Partials        1        1              

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