Exposing getNodeRect

[cuginoAle]

With this PR d3-fg will expose the getNodeRect public method.
It accepts the node data object and it returns an object containing the node position and size (or null if no node is found).
The returned object has now the following shape:
{x, y, width, height}

This has been updated to match the object shape returned by getClientBoundingRect.

This method is useful when we need to overlap external components over a particular frame, like the following screenshot demonstrates:

[goto-bus-stop]

data.id is a Flame-specific thing, 0x doesn't have it. (The distinction is definitely not very clear…) Could we pass in the node.data object instead? Existing APIs use that, like the chart.zoom(d) method.

We should document the properties that a d3-fg data object always has in the readme so we don't have to rely on people knowing and noticing which things are specific to 0x and Flame. I think those would be the same properties more or less as returned by the ticksToTree method in 0x. Flame adds id and a few more, and 0x has IDs but it uses a Map to store data/id pairs instead of adding an id property.

[cuginoAle]

@goto-bus-stop Good to know, thanks for clarifying this!
Sorted now.

[AlanSl]

LGTM