Consider combining myaddon and --myaddon

[Rich-Harris]

At the moment if you do npx sv add myaddon you're asked to specify the options for myaddon, but there's also npx sv add --myaddon which doesn't prompt for options; it just selects defaults for you.

This distinction feels a little confusing to me. What if the prompt appears whenever an add-on has options and there's no =? And if the user specifies = but required options are missing, bail. So:

# prompt for drizzle options
npx sv add drizzle

# install drizzle with postgresql and postgres.js
npx sv add drizzle=postgresql,postgres.js

# bail, because required `client` option is missing (`docker` is not required in this example)
npx sv add drizzle=postgresql