feat(core): expanded attach

[CodyJasonBennett]

Expands the attach prop by accepting an attach function in addition to a string. This is 1-1 with R3F v8's unified attach API.

// will attach to parent.target
<parent>
  <child attach="target" />
</parent>

// will attach to parent.target.subtarget
<parent>
  <child attach="target-subtarget" />
</parent>

// will attach to parent.target[0] and parent.target[1], creates an array if parent.target is undefined
// useful for attaching multiple elements to an array
<parent>
  <child attach="target-0" />
  <child attach="target-1" />
</parent>

// will call child.setParent on mount and child.removeChild unmount
<parent>
  <child
    attach={(parent, self) => {
      self.setParent(parent)
      // remove on unmount
      return () => parent.removeChild(self)
    }}
  />
</parent>