Add device support for Samsung dehumidifiers (issue #88)

The AY18CG7500GED dehumidifier (modelNum TP1X_DA_AC_DHM_01001_0000)
shares the DA_AC_ board family with the room-AC models but carries the
'_DHM_' token instead of '_RAC_'/'_PRAC_'/'_WAC_', so it fell into the
unknown-device-type fallback. Add a new dehumidifier registry, routed
via a '_DHM_' modelNum fallback rule, distinct from airconditioner
since target humidity (not temperature) is the primary control and
there's no climate composite.

Reuses airconditioner.py's AUTO_CLEAN/AIR_FILTER/MUTE_ONCE capabilities
directly (identical resource shapes on the shared board family). Adds
a new humidity sensor + target-humidity number pair and an
operating-mode select. Per the adding-device-support skill's
"never hard-code the one dump's values" rule, the target-humidity
number has no hardcoded min/max (falls back to HA's own 0-100 default
for a percentage field) and reads its step live from the device's own
`increment` field rather than a spec-sheet-derived constant. The
operating-mode select's options come live from supportedModes.

/mode/convenient/vs/0 is left unmodeled: only supportedModes is present
on this dump, with no live current-value field to confirm a read/write
contract.

Confirmed against the issue #88 diagnostics dump with zero unbound
hrefs. Updates the README's supported-appliance-types table for the
new device type.

README.md

@@ -23,6 +23,7 @@ Your state stays on your LAN: HA talks to the appliance over a direct DTLS sessi
 |---|---|
 | Air conditioner | `by_type/airconditioner.py` |
 | Air purifier | `by_type/air_purifier.py` |
+| Dehumidifier | `by_type/dehumidifier.py` |
 | Dryer | `by_type/dryer.py` |
 | Oven | `by_type/oven.py` |
 | Cooktop (read-only burner status) | `by_type/cooktop.py` |