Fixes shellcheck errors in script/*
Adds a couple trailing newlines.
Adds `script/shellcheck-scripts` and associated CI machinery.
Current set ultra-conservative, does not output warnings, only errors.
Also, move postgrest service + deployment into a separate manifest
file, which is deployed via a separate script, `deploy-postgrest`.
This way, we don't have unused postgrest instances in preview and
nightly, since those use the prod database.
Co-authored-by: Conrad <conrad@zed.dev>