allow kv_namespaces in toml

[EverlastingBugstopper]

fixes #1408 -

when KV namespace support was initially added to Wrangler, we documented using `kv-namespaces` in `wrangler.toml`. Unfortunately, the `-` was not consistent with other fields such as `zone_id` and `account_id`, so the decision was made to allow both `kv-namespaces` and `kv_namespaces`. When this change was introduced, it worked with top level `kv_namespaces` entries, but not in environments. This is now fixed! You can now use `kv_namespaces` everywhere you can use `kv-namespaces`.