fix: folding vehicles fold up into proper item

[shmakota]

Checklist

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Purpose of change

Allows folding a vehicle to work (presumably) as intended, and return the specified item. Previously, if someone added a new item "skateboard" and refolded it after placing it down, it would return a folding_bicycle item. Now it will return the "skateboard" item. Also changes name of the folding wheelchair, and the way folding vehicle names are created. Previously it would fold into "folded Folding wheelchair" now just into "wheelchair."

Describe the solution

Unhardcode the string "folding_bicycle" and make it return itype_id instead when specifying the folding_veh_item.

Describe alternatives you've considered

Testing

Spawned unicycle and wheelchair, they work as expected upon folding/unfolding. Spawned a folding_bicycle item, works as expected. Created a test item folding_flatbed which unfolds into a flatbed truck, and upon it being refolded it correctly returns the folding_flatbed.

Additional context

[yay855]

Looks good; I'll test it myself once the checks come through, but it should work easily enough.

[RobbieNeko]

Builds on my machine (Linux)!
Tested folding bike and wheelchair, both indeed behave as expected!

[RoyalFox2140]

What are you, lurking in the dev chat secretly?

[RobbieNeko]

Checks passed, now for confirmation that it doesn't magically bork itself on Windows since this is a C++ change ^-^

[chaosvolt]

1. Compiled and load-tested.
2. Spawned in a folding bike, deploying it and repacking it causes no odd behaviors.
3. Redeploying it afterward also seems to work as intended.
4. Spawned in a wheelchair, it folds up and deploys as expected, still preserves its part damage and ability to modify the item.
5. Edited folding bike item to spawn a car instead, it...somehow folds up when examined and turns into a folding bicycle again. I only changed what the item spawns when unfolded and not the name or anything, so this is...probably expected behavior?