Add a predicate method to each node

This allows the AST to be searched via a predicate method rather than
hardcoding the class name and doing is_a? checks.  For example, rather
than:

```
ast.grep(Psych::Nodes::Scalar).each do |node|
  # .. do something
end
```

Now you can do:

```
ast.find_all(&:scalar?).each do |node|
  # .. do something
end
```

Your code no longer needs to know the exact class used in the AST.

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