[camera_web] Add support for a flash mode

[bselwe]

Adds support for a flash mode to the web camera platform.

• Added setFlashMode to Camera.
  • Saves the flash mode in the camera.
  • Enables the torch mode video constraint if the flash mode is torch (otherwise disables).
  • Throws a CameraWebException if the torch mode is not supported in the current browser or the camera has not been initialized or started.
• The camera stream can now be accessed through the Camera.stream property.
• Updated takePicture in Camera.
  • Enables the torch mode video constraint when taking a picture if the flash mode is either auto or always.
• Added setFlashMode implementation to the camera plugin.
  • Calls setFlashMode on the camera with the given id.

Part of flutter/flutter#45297.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[bselwe]

@ditman @felangel

[ditman]

Let's go!

[ditman]

I think this is going to be slightly odd behavior, but I'm not sure how this can be implemented, other than the highly experimental fillLightMode constraint.

Can we add a comment around here explaining why takePicture does slightly different things than setFlashMode? Somebody tracing this will see:

setFlashMode set to auto (explicitly DOESN'T SET torchMode -> takePicture sets torchMode to true

[bselwe]

Good point, will add a comment for that!

[bselwe]

Added in #4227.