Simplify SwiftBasicFormat by introducing structural information of the syntax tree as static properties

[ahoppen]

The core idea here is that we a static property structure to all the syntax nodes, which allows us to query the statically known structure of the syntax tree. That way BasicFormat can have a list of all the children it should indent or put on a new line, reducing its implementation from 3,000 to 300 lines.

structure will also be useful in a variety of other use cases:

• We can use it to improve debugInitCall to insert upcast conversions it is currently missing because it can use childrenKeyPaths to query the static variable type of a child
• I have a feeling that childNameForDiagnostics could be implemented more cleanly on top of childrenKeyPaths
• I will try to implement keyword completion on top of this infrastructur
• It could allow us to specify where nodes should be inserted into a syntax tree. Currently e.g. Fix-Its only support replacements.

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test