Add isSingleBlock() to Bisected for distinguishing single-block bisected types

[4everTheOne]

This PR introduces a new method to the org.bukkit.block.data.Bisected interface:

default boolean isSingleBlock() {
    return false;
}

Purpose

The method allows plugins and internal logic to differentiate between bisected blocks that occupy a single block space (e.g., trapdoors, stairs) and those that span two vertical blocks (e.g., doors, tall plants).

Behavior

• Returns true for trapdoor and stairs blocks
• Returns false for other blocks like doors and double-height plants