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This adds a setting to easily set the web renderer at the User Setting or Workspace Setting level. It will be injected into flutter run commands (launched via the debugger) and flutter build web tasks (from the Flutter task contributions) unless they already have this in their args.

TODO:

• Confirm which version of Flutter we should pass this for (currently set to >= v1.25.0-0)
• Decide whether we need both default and auto or if we could fold them together (eg. only have auto and always pass it, or only have default and leave auto as something Flutter does when using its default)
• Add a link to docs describing the renderers in more detail?

Fixes #2999.