CapturePatternNode

Right now, when you capture an expression into a variable in pattern matching, we get a binary node. For example,

```ruby
case foo
in Bar => baz
end
```

Instead, we should create a new node to represent this pattern. YARP calls this `AsPatternNode` but I don't like the name and am going to rename it to `CapturePatternNode` to be more clear.