std.zig.Ast: add printNode

[mateusz834]

This change adds a function to print a visual AST representation.
It makes the experience better while learning/debugging stuff related to the Zig AST.

[mateusz834]

I have developed such utility while looking around and learning the Zig AST. It seems pretty useful, so I sent a PR.

I wonder whether we should wire this thing up in ast-check, just like -t prints the ZIR, we could have some option that prints the AST.

[mateusz834]

You might wonder why the tree is indented using ". " (a dot followed by two spaces), which at first glance seems weird.

This comes from Go’s go/ast (see print.go).
The reason is practical: using a visible dot makes it easier to spot the end braces and distinguish indentation levels in the printed tree