Initial share provider API and UI (#182999)

* Formalize share provider API

* i18n.resources.json

* Don't introduce a generic Success dialog severity

extensions/github/src/typings/vscode.proposed.shareProvider.d.ts

@@ -0,0 +1,29 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+// https://github.com/microsoft/vscode/issues/176316
+
+declare module 'vscode' {
+	export interface TreeItem {
+		shareableItem?: ShareableItem;
+	}
+
+	export interface ShareableItem {
+		resourceUri: Uri;
+		selection?: Range;
+	}
+
+	export interface ShareProvider {
+		readonly id: string;
+		readonly label: string;
+		readonly priority: number;
+
+		provideShare(item: ShareableItem, token: CancellationToken): ProviderResult<Uri>;
+	}
+
+	export namespace window {
+		export function registerShareProvider(selector: DocumentSelector, provider: ShareProvider): Disposable;
+	}
+}