Add sub-categories for crossings in option4

[robinlovelace-ate]

In option3 the crossings are as follows:

schema/option3/crossings.json

Lines 7 to 12 in 202a377

	"enum": [
	"Grade separated",
	"Signalised",
	"Priority",
	"Uncontrolled"
	]

It would be good to include the sub-categories from #19 (comment) being

• Grade separated (bridge, tunnel, other?)
• Signalised (pegasus, puffin & toucan)
• Priority (zebra, school (?) and parallel)
• Uncontrolled (refuges, dropped kerbs, tactiles)

[robinlovelace-ate]

I think it's doable and fairly standard with the json-editor JS package currently used for this: https://github.com/json-editor/json-editor#dependencies

[robinlovelace-ate]

Latest option: https://acteng.github.io/schema/option3/