Refactor beeper (#13492)

Refactor beeper (#13492)

* Beeper - improve/fix beeper sequencing

- beeperSequenceAdvance is used to advance beeperPos
- sequences starting with 0 (delay first) are handled correctly
- 'empty' beeper states are supressed
- leds do blink beeper sequence, even when beeper is silenced

* Beeper - refactor beep_multiBeeps

- simpler length definition
- helpper function to simplify beep generation

* Beeper - refactor beeperTable

Refuse modes where beeperTableEntry->sequence is NULL
Rest of code is refactored without functional changes

* Beeper - improve micros() overflow handling + minor refactoring

- beeperNextToggleTime==0 when not enabled
- lastDshotBeaconCommandTimeUs is updated during DSHOT_BEACON_GUARD_DELAY_US phase
- DSHOT_BEACON_ALLOWED_MODES instead of explicit tests

* Beeper - minor refactor

- improve comments
- add STATIC_ASSERT
- reformat some code
- move #ifdef to better follow semantic structure
- return unsigned from BEEPER_GET_FLAG

* fixup! Beeper - minor refactor

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Co-authored-by: Petr Ledvina <ledvinap@hp124.ekotip.cz>