feat: Support and test Sentry.sendFeedback

[timfish]

Closes #834

This PR ensures that feedback can be sent from the renderer process and adds a test to ensure the envelopes correctly get passed through the main process.

[billyvg]

I'm pretty sure that the backend will ignore any additional contexts, but will double check for you.

[billyvg]

Also as a headsup, we're looking at consolidating captureUserFeedback with sendFeedback (and likely killing the latter API) for v8

[timfish]

we're looking at consolidating captureUserFeedback with sendFeedback

Yep no problem, will update all these tests when it happens.

I think the main difference (in features/behaviour at least!) is that sendFeedback doesn't have a way to attach a related event id?

[billyvg]

I think the main difference (in features/behaviour at least!) is that sendFeedback doesn't have a way to attach a related event id?

Yep exactly