feat(react): add useModel hook

[manzt]

Adds a hook to grab the model. Can be useful to subscribe to custom messages:

import * as React from "react";
import { createRender, useModel } from "@anywidget/react";

export let render = createRender(() => {
  let model = useModel();
  let [data, setData] = React.useState(null);

  React.useEffect(() => {
    let callback = (msg) => setData(msg);
    model.on("msg:custom", callback);
    return () => model.off("msg:custom", callback);
  }, [model]);

  if (!data) return null;
  return <pre>JSON.stringify(data)</pre>;
});

[netlify]

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[codecov]

Codecov Report

Merging #200 (bf06366) into main (c48b829) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #200   +/-   ##
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  Coverage   98.11%   98.11%           
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  Files           8        8           
  Lines         424      424           
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  Hits          416      416           
  Misses          8        8           

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