API for expand and collapse

[josdejong]

Implements a solution for #249

Initially I've tried to make the existing expand method more flexible, but it turns out there are a couple different use cases that can't be supported in an easy way by a single callback function, and the expand and collapse functionality is not symmetric. So, I ended up implementing 4 use cases in two methods:

// expand all nodes along the path, and update the visible sections of an array if needed 
// (for example used by `editor.scrollTo`)
editor.expand(path: JSONPath)

// expand all nodes along the path, and recursively expand part or all of the nested children inside the node 
// depending on the callback (used on Ctrl+Click on an expand button)
editor.expand(path: JSONPath, callback: (relativePath: JSONPath) => boolean)

// collapse a single node at the path 
// does not collapse parent nodes along the path, and does not collapse nested children
editor.collapse(path: JSONPath)

// collapse a single node at the path and collapse all of its children when recursive is true
editor.collapse(path: JSONPath, recursive: boolean)

Note that this is a breaking change, and will be merged in the v1 branch.