preview/dev namespaces

[EverlastingBugstopper]

fixes #1032

adds preview_id field to items in kv_namespaces that must be provided in order to preview a worker that has kv namespaces. this will prevent folks from accidentally screwing up their production data when using wrangler dev.

also adds --preview to kv commands in order to interact with them instead of production namespaces.

if preview_id and id are same:

$ wrangler preview
💁  JavaScript project found. Skipping unnecessary build!
⚠️  Specifying the same KV namespace ID for both preview and production sessions may cause bugs in your production worker! Proceed with caution.
👷  Your Worker responded with: key: production value

if no preview_id is specified

$ wrangler preview
💁  JavaScript project found. Skipping unnecessary build!
Error: In order to preview a worker with KV namespaces, you must designate a preview_id for each KV namespace you'd like to preview.

if preview_id and id are different:

$ wrangler preview
💁  JavaScript project found. Skipping unnecessary build!
👷  Your Worker responded with: key: preview value

[ashleymichal]

i think this has some lovely ideas; i'd like to see what happens if you pass preview bool to get_target instead of adding a preview_kv_namespaces fn to Target, and retain the plain Vec<KvNamespace> on Target.

[ashleymichal]

just remove the old comment and 🚀