feat: expose extension roots as Observable

Author: sqs

src/extension/api.ts

@@ -22,7 +22,7 @@ export interface SourcegraphExtensionAPI<C = Settings> {
      * TODO(sqs): Remove the strict association with the single initial root (support multi-root, changing roots,
      * etc.).
      */
-    root: URI | null
+    roots: Observable<URI[]>
 
     /**
      * The configuration settings from the client.

src/extension/extension.ts

@@ -1,4 +1,4 @@
-import { Subscription } from 'rxjs'
+import { BehaviorSubject, Subscription } from 'rxjs'
 import { createMessageConnection, Logger, MessageConnection, MessageTransports } from '../jsonrpc2/connection'
 import {
     ConfigurationCascade,
@@ -28,6 +28,7 @@ class ExtensionHandle<C> implements SourcegraphExtensionAPI<C> {
     constructor(public readonly rawConnection: MessageConnection, public readonly initializeParams: InitializeParams) {
         this.subscription.add(this.rawConnection)
 
+        this.roots = new BehaviorSubject<URI[]>(initializeParams.root ? [initializeParams.root] : [])
         this.configuration = createExtConfiguration<C>(
             this,
             initializeParams.configurationCascade as ConfigurationCascade<C>