VolumeScroll: fix broken start() and modernize modifier matching

[TowyTowy]

Fixes #341

VolumeScroll errored on start() and its event tap callback threw attempt to call a nil value (method 'sameMods'). The tap callback resolved helper methods through a self that was easily wrong (the documented signature did not show the mods parameter, so dot-calls were common — which also let the tap object be garbage-collected, explaining the "tap eventually stops receiving events" symptom), and the hand-rolled sameMods/tableLength comparison relied on a synthetic scroll event created in :init() just to capture flags.

• Match modifiers with event:getFlags():containExactly(self.mods), removing sameMods(), tableLength() and the synthetic event in :init()
• Make :start() work with no arguments (defaults to { "cmd" }) and document the optional mods parameter in the signature, with a colon-call note and usage example
• Guard against output devices with no volume control and clamp volume to 0..100
• Make :stop() safe to call before :start()
• Return self from init/start/stop per SPOONS.md chaining convention

Validated with luac -p, the CI docstring linter (build_docs.py -l), and a mocked-hs logic trace covering both error reproes from the issue.