Add submission as submit event payload

[brontolosone]

Towards #23
Towards #290

Requirement of getodk/central-frontend#1125

With this, the submission becomes available to a "host application" as in the payload of the Vue event that is emitted when the submit button is used.

I have verified this PR works in these browsers (latest versions):

• Chrome
• Firefox
• Safari (macOS)
• Safari (iOS)
• Not applicable

What else has been done to verify that this works as intended?

Why is this the best possible solution? Were any other approaches considered?

Instead of having to await in the event handler, we could instead await right here in Webforms-land so that the event handling code doesn't have to. Perhaps that's easier for host application writers. I'm happy with it either way.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Do we need any specific form for testing your changes? If so, please attach one.

What's changed

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Latest commit: a5c90e9

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