Add a method to render multiple output formats

[mdaines]

See #259

Adds a renderFormats() method which accepts an array of output formats to render. This avoids redundant parsing and layout when rendering the same graph with multiple output formats. The use case in the linked issue is rendering an SVG and a corresponding image map.

Usage:

const result = viz.renderFormats("digraph { a -> b [href=\"https://example.com\"] }", ["svg", "cmapx"], { engine: "neato" });
result.output // => { "svg": ..., "cmapx": ... }

If rendering is successful, result.output is an object containing the output for each format, keyed by format.

Results from a basic performance test. This compares separate calls to render() with one call to renderFormats():

$ node performance-multiple.mjs 
render, 100 nodes, 10 edges: 1863 in 5.00 s, 372.54 calls/s
render, 1000 nodes, 50 edges: 96 in 5.04 s, 19.05 calls/s
render, 1000 nodes, 500 edges: 60 in 5.01 s, 11.98 calls/s
render, 1000 nodes, 1000 edges: 11 in 5.37 s, 2.05 calls/s
renderFormats, 100 nodes, 10 edges: 2706 in 5.00 s, 541.05 calls/s
renderFormats, 1000 nodes, 50 edges: 162 in 5.02 s, 32.24 calls/s
renderFormats, 1000 nodes, 500 edges: 102 in 5.04 s, 20.25 calls/s
renderFormats, 1000 nodes, 1000 edges: 20 in 5.11 s, 3.91 calls/s