Add back Azure Resource Explorer extension with updated build script (#2805)

* Revert "Revert "Port the Azure Resource Explorer extension to core." (#2770)"

This reverts commit 210447cd37.

* WIP1

* Add custom build task for azurecore extension

* Fix azurecore output path for macOS build

* Fix linux gulp task name

extensions/azurecore/src/constants.ts

@@ -0,0 +1,8 @@
+'use strict';
+
+export const extensionConfigSectionName = 'azure';
+export const ViewType = 'view';
+
+export enum BuiltInCommands {
+	SetContext = 'setContext'
+}