feat: support elastic tab stops in formatter

[DavePearce]

Summary

The elastic tab stops algorithm is an approach for getting better "alignment" when formatting source code. See this article for reference. This algorithm is used by go fmt and the aim of this issue is to adopt something similar.

Examples

Examples from go fmt:

var (
     x = 1       // a comment
     y = f(args) // another comment
)

var (
    x      = 1
    anchor = f(args)
)

var (
	x      uint32
	anchor uint64
)