Prior to this change, bicycles had only a 3% chance among the vehicle
pool (less than half that of armored vehicles) on city streets.
This seemed way too low, so I wanted to give bikes a fairer chance.

Fixes CleverRaven#37451

This commit changes three vehicle groups, including:

city_vehicles:
- Bicycle equally likely as the standard car,
  or twice as likely as a Hippie Van.
- Dirt bike as likely as a Hatchback or Hippie Van
- Electric bicycle as likely as a Beetle or RV

suburban_home:
- Bicycle as likely as standard car
- Dirt bike as likely as SUV
- Electric bicycle possible, as likely as scooter or sports car

parkinglot:
- Bicycle as likely as car or flatbed truck
- Dirt bike possible, as likely as hatchback or motorcycle

Based on PR feedback from other devs/players, I'm dialing back the
bicycle spawn frequency significantly from my previous commit.

The total adjustment now is that bicycles are roughly 1.5 - 2 times as
likely to appear in city streets, suburban homes, and parking lots, as
they are in the current experimental branch.

Adjustments made in this commit include:

In "city_vehicles" (i.e. in streets everywhere):

- "bicycle" weight is 800 (originally 400), making them
  rarer than "car", "car_hatch", "ambulance", or "hippie_van",
  but more likely than "beetle", "car_mini", "bus", "rv", or "4x4_car".
  Same chance of spawning as a "pickup" or "suv".

- "bicycle_dirt" weight is 200 (originally 0, impossible), making them
  as likely as "suv_electric", "motorcycle", "truck_swat", or "fire_engine"

- "bicycle_electric" weight is back to 100 (originally 100)

In "suburban_home":

- "bicycle" weight is now 200 (originally 150),
  same chance as "4x4_car" or "motorcycle"

- "bicycle_dirt" weight is now 150 (originally 125),
  same chance as "car_anmlcmpt"

And in "parkinglot":

- "bicycle" weight left at 1000 (originally 500)

- "bicycle_dirt" removed again (back how it was before)

Partially fixes CleverRaven#37451

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 CleverRaven#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".

My JSON failed linting, now it should work

To see if that is enough lint removal