Add a bike shed to house08 and house_patio

To get a few more pedal-powered bicycles and parts into the world in a
less conspicuous way than simply spamming them onto the city streets

Fixes #37451

As suggested by @I-am-Erk, this adds a 6x6 nestable mapgen module
containing a simple metal bike shed. It's modeled after the 6x6_junk
shed, with flimsier metal walls, but still having locked (pickable)
metal doors, so it's not a total smash-and-grab.

Inside the shed is a 50% chance of finding a "bikeshop" vehicle, most
often a "bicycle", "bicycle_dirt", or "bicycle_electric", but with some
chance of "tandem", "scooter", "unicycle" etc.

It's furnished with a workbench and metal shelf, across which there is a
low (10%) chance of seeing 1-3 "bikeshop_tools", usually a bike part
(wheel, horn, foot crank, saddle) but sometimes such goodies as a wrench,
screwdriver set, duct tape, or even (very low chance) a toolbox!

I found two houses where this shed seems to fit well with the layout
("house08" and "house_patio"), and included the new bike shed with the
same probability as "shed_6x6_woodworker".

data/json/mapgen/house/house08.json

@@ -58,6 +58,7 @@
             [ "greenhouse_6x6_vegetable", 25 ],
             [ "pond_6x6", 25 ],
             [ "shed_6x6_junk", 25 ],
+            [ "shed_6x6_bike", 25 ],
             [ "shed_6x6_woodworker", 25 ]
           ],
           "x": 15,

data/json/mapgen/house/house_patio.json

@@ -69,6 +69,7 @@
             [ "greenhouse_6x6_vegetable", 25 ],
             [ "pond_6x6", 20 ],
             [ "shed_6x6_junk", 25 ],
+            [ "shed_6x6_bike", 20 ],
             [ "shed_6x6_woodworker", 20 ]
           ],
           "x": 1,

data/json/mapgen/nested/house_nested.json

@@ -1638,6 +1638,38 @@
       ]
     }
   },
+  {
+    "type": "mapgen",
+    "method": "json",
+    "//": "a nested map for lawns",
+    "nested_mapgen_id": "shed_6x6_bike",
+    "object": {
+      "mapgensize": [ 6, 6 ],
+      "rotation": [ 0, 3 ],
+      "rows": [
+        "||||||",
+        "|    +",
+        "|   B|",
+        "|   B|",
+        "+   S|",
+        "||||||"
+      ],
+      "terrain": {
+        "|": "t_scrap_wall",
+        "+": "t_door_metal_pickable",
+        " ": "t_thconc_floor",
+        "S": "t_thconc_floor",
+        "B": "t_thconc_floor"
+      },
+      "furniture": {
+        "T": "f_trashcan",
+        "S": "f_utility_shelf",
+        "B": "f_workbench"
+      },
+      "place_vehicles": [ { "vehicle": "bikeshop", "x": 1, "y": 2, "rotation": 270, "chance": 50 } ],
+      "place_items": [ { "item": "bikeshop_tools", "x": 4, "y": [ 2, 4 ], "chance": 10, "repeat": [ 1, 3 ] } ]
+    }
+  },
   {
     "type": "mapgen",
     "method": "json",