[Feature] IME support

Right now our IME support is limited. We insert text, but do not allow for testing complicated scenarios with composition events.

The Web api: https://developer.mozilla.org/en-US/docs/Web/API/CompositionEvent
The Mac api: https://developer.apple.com/documentation/appkit/nstextinputclient?language=objc
The Chromium mojo api: https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/public/mojom/input/input_handler.mojom

This request comes from @trueadm on twitter: https://twitter.com/trueadm/status/1369332999242792967

My initial api proposal:

await page.keyboard.startComposition(start, end); // optional start and end
await page.keyboard.updateComposition('foo');
await page.keyboard.updateComposition('bar');
await page.keyboard.press('A'); // what should this do? Send keyboard events without generating text?
await page.keyboard.endComposition('baz'); // optional ending text

The only time I've seen composition events used in the wild is in CodeMirror to draw a synthetic composition highlight. It would be helpful to have some better use cases here to make sure that the api handles them without growing too large.

• Does the composition state need to be inspected, or just controlled? That is, do we need keyboard.compositionRange()?
• Do we instead need a high level composition api. Like keyboard.typeWithComposition("è") types AltGr+` followed by e?
• Do we need be able to trigger composition via keys. Right now our keyboard api largely ignores dead keys that would start composition.