Add a way to read all enabled extensions across all extension hosts

vscode.extensions.all or vscode.extensions.getExtension will only return extensions from the current extension host. The root cause for this is that the return type IExtension<T> exposes the .exports property, which represents the extension’s exported API. Since extensions can export synchronous APIs, we can’t automatically proxy it to make it available to other extension hosts.

Also, making vscode.extensions.getExtension return an extension running on a different extension host, has the potential to cause breakage in existing extensions.

We could add a new function to be able to get to extensions that are running on other extension hosts.

This came up before e.g. in #82633

cc @jrieken

[aeschli]

The main motivation for me is to get access to the manifests of all extensions across extension hosts to find contributions such as jsonValidation.

The same issue came up with the new anycode contributions and the htmlLanguage contribution were exploring in #146731.

[DanTup]

@jrieken perfect, thanks!