Add shift-left-click on the sign as a shortcut to destroy the gate

Same permission check as /sg destroy (owner with stargate.destroy, or
stargate.admin). Also breaks the physical sign block via
breakNaturally() so it drops like a normal mined sign instead of just
vanishing, matching what happens when StructureProtectListener reacts
to an actual BlockBreakEvent on the sign.

README.md

@@ -66,6 +66,9 @@ material, unsealed interior, too far from the sign, etc.
 - **Left-click** the sign: dials the shown destination - chevrons light in sequence,
   then the interior becomes the event horizon (`gate.iris-open-material`, default
   water) and travel opens up. It auto-closes after `dialing.open-seconds`.
+- **Shift + left-click** the sign: destroys the gate on the spot (same permission
+  check as `/sg destroy` - owner or `stargate.admin`), breaking the sign as if
+  you'd mined it. Quicker than looking at the sign and typing the command.
 - Travel is **one-way**: only the gate you dialed *from* can send anything anywhere.
   Walking (or wandering, or drifting) into its event horizon teleports whatever
   entered to the destination, arriving just clear of its iris (never inside it) and