chore(router): simplify Submission type

[MichaelDeBoey]

Follow-up of #10413

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[brophdawg11]

I'm ok with this duplication for now. I can see a future where we narrow the enctypes appropriately across the Submission union since it can only be encType: "application/json" when json is present, etc. I also think that formMethod can only technically be MutationFormMethod for json/text submissions since they can't be converted to URLSearchParams like FormData can. So if we ever tackle those we'd have to undo this DRY change.