Export ExtensionRuntime (or LanguageClient) for other extensions

[BurstingF]

This change would export the ExtensionRuntime object that is created such that other extensions can access it.
This would, for example, allow other extensions to depend on this extension and use the LanguageClient it creates to send requests without the need to start their own LanguageClient.

With the current changes other extensions could access the LanguageClient like this:

const client: LanguageClient = vscode.extensions.getExtension('dafny-lang.ide-vscode')?.exports.client;
client.sendRequest(...)

Note: Perhaps if exporting ExtensionRuntime is a bit too broad, only the LanguageClient could be exported instead.

[keyboardDrummer]

Seems good, thanks