`deno add npm:foo` adds dependency to `package.json` instead of `deno.jsonc`

[johnstonmatt]

When a new dependency is installed using deno add npm:cowsay the entry is added to the package.json file instead of the more idiomatic deno.jsonc. It's easily worked around by just adding the entries to deno.jsonc instead, just a slightly rough edge.

cc @jacob-ebey