Allow custom integrations to associate themselves with the client:only directive

[RohanHart]

Summary

clientOnlyValues is a static Set containing only official integrations. Allowing a custom integration to register themselves with this under some name would provide full client:only support.

Background & Motivation

Client-only rendering with @analogjs/astro-angular or any other custom integration will not work when there are multiple renderers in the Astro configuration.

Aside from the the officially supported renderes, as defined in the clientOnlyValues, there is a fallback to guessing based on the component extension. This will not work well for Angular as their default naming scheme will result in component and it makes no sense for a custom integration to call itself that.

Proposal

• When a custom integration provides a clientEntrypoint that includes registration of a client:only renderer under the integrations name.
• Optionally, extend AstroRenderer to allow a short alias for the renderer so that the directive can be specified more simply, eg., client:only="angular"

[matthewp]

Seems reasonable to me.