Feature Request: Allow custom domains that do not require .local TLD/extension

### What

The custom domain functionality isn't truly custom. You have to use the .local TLD

### Why

Totally a personal preference, when I am developing plugins locally I'll spin up an install using the following format https://{namespace}.plugin in other local dev setups that do allow this. This is to differentiate between sites, plugins, apps, etc. Just a quality of life item

### How

Start by locating the custom-domain configuration and validation entry point in WordPress Studio, then review how the current .local requirement is enforced. Confirm the desired behavior by configuring a domain such as {namespace}.plugin and verifying that the local site starts and remains reachable without requiring the .local suffix.

Written by the indexing model from the issue text.