wspr: Add VDL files for browspr and js communication

With guessType changing, we should rely more on VDL to ensure we send
the right information between the two.

valconv was removed, but since channel takes generic input from JS,
we still have to keep interface{} around and do some type casting.

Not included: Tests for the new error cases when type casting.

vom: https://vanadium-review.googlesource.com/#/c/2461/
veyron.js: https://vanadium-review.googlesource.com/#/c/2462/
viz: https://vanadium-review.googlesource.com/#/c/2511/
wsprd: https://vanadium-review.googlesource.com/2595
playground: https://vanadium-review.googlesource.com/#/c/2714/

MultiPart: 4/5
Change-Id: Ie0b4a8303a9ae0bc8943cb855c25fea4f6799daa
19 files changed
tree: fe6c7a6c6550c121d88d5f93d416c4d1dd18c8ee
  1. services/