This PR introduces a new option for the `@uppy/webcam` plugin called `preferredMimeType` which defaults as `null`.

If a value is passed, the browser's `MediaRecorder` implementation checks if the `preferredMimeType` is supported, and if it thinks it is, will attempt to record using the `preferredMimeType` rather than the browser default.

If the `preferredMimeType` is unsupported, the browser will fall back to using its default recording mime type.

This allows, for example, a user to pass `preferredMimeType: "video/webm"` during the `@uppy/webcam` plugin construction which will instruct the browser to record to a `.webm` container.

Specific codecs may also be passed here. For example, `video/webm;codecs=vp8` may still parse as valid by the browser.

## Limitations
This does not currently change the mime type for captured images on line 282 (`image/jpeg`) and will only pass the `preferredMimeType` to the browser `MediaRecorder` API.

### Reference
https://developer.mozilla.org/en-US/docs/Web/API/MediaRecorder/MediaRecorder
https://developer.mozilla.org/en-US/docs/Web/API/MediaRecorder/isTypeSupported
https://cs.chromium.org/chromium/src/third_party/blink/web_tests/fast/mediarecorder/MediaRecorder-isTypeSupported.html?q=MediaRecorder-isTypeSupported&dr
https://www.webmproject.org/about/faq/