Narrow BufferAttribute's array to TypedArray

[Methuselah96]

Why

Resolves #35. This allows code like this snippet from the misc_exporter_ply.html example to work:

const geometry = new THREE.BoxGeometry();

// color vertices based on vertex positions
const colors = geometry.getAttribute("position").array.slice();
for (let i = 0, l = colors.length; i < l; i++) {
  if (colors[i] > 0) colors[i] = 0.5;
  else colors[i] = 0;
}

What

Introduce a TypedArray type that is the union of all the typed arrays. This excludes BigInt64Array and BigUint64Array so that we don't have to change the ES version we're targeting. The better solution would be for TypeScript to provide a TypedArray interface which is discussed here.

Checklist

• Checked the target branch (current goes master, next goes dev)
• Added myself to contributors table
• Ready to be merged