More convenient teleport for `/ac mine`

[3TUSK]

Possible options:

• Teleport to the highest Y where there is a solid block (i.e. according to height map)
• Search for air block near the center of the area
• Allow area owner to set a "teleport point" and use that

[3TUSK]

ea2afc8 implements the option 1 (use height map as reference). We will see how it works out.