Improve indentation behavior

[jaybosamiya]

Currently we simply directly use the Rust indenter; might be good to have something specific for Verus, so that <tab> works better

[jaybosamiya]

Potential temporary fallback: explicitly disable electric-indent-mode, since we don't yet have a full Verus indenter; at least should prevent the annoyances that arise when hitting return

[jaybosamiya-ms]

Hmm, now that we've had verusfmt for a while, we could consider having it enabled on save, or at minimum, use it in the background as a potential hinting service