Capitalize the E in contenteditable

Turns out `attr "contenteditable" "true"` has no effect, but `attr "contentEditable" "true"` works like a charm!

This PR just updates the typed convenience function. Technically this is a breaking API change, but that can be trivially avoided by leaving the function name as `contenteditable` but having it emit `"contentEditable"`. I figured keeping the name and what it emits in sync was worth the breaking change, but happy to go the other way too.