Implement editor tools for working with RoadPoints

[TheDuckCow]

Right now, the setup and use of roads is very manual and cumbersome. We should add some tools to make it convenient to work with inside the Godot Editor (without sacrificing the ability to use it for procedural use cases too).

Some key functionality that will be required (these can be split up into additional Issues as appropriate when ready):

• [x] Being able to one-click add a road network setup, with a container for road points. Should start off by adding a single road point
• [x] Being able to use some kind of button to extend or draw the next road point. Some UX discussion should happen to see about the combination of the most "godot" way to do this while still being the best possible user experience. Adding / extending should inherent all settings from the prior node, such as number of lanes and texturing and material reference.
• [x] Be able to insert RoadPoints in the middle of RoadSegments (subdivide), inheriting the appropriate interpolation properties at that position
• [x] Be able to delete RoadPoints, automatically connecting the RoadPoints on either side (and doing best effort to update bezier handles to fill the space in the middle)
• [x] Controls to change the number and position of lanes on a single RoadPoint
• [x] A custom panel to make it easier to apply bulk changes to all selected road points (and to select RoadPoints within a network); common use case will be changing all RoadPoints to have the same lane width, or even number of lanes.

[TheDuckCow]

All tasks have been split out; tick boxes above are not to imply that these activities have been completed. See the linked open tasks above for reference.