Get communication working between extension and webview; add shared data types

src/shared/ExtensionMessage.ts

@@ -0,0 +1,8 @@
+// type that represents json data that is sent from extension to webview, called ExtensionMessage and has 'type' enum which can be 'plusButtonTapped' or 'settingsButtonTapped' or 'hello'
+
+// webview will hold state
+export interface ExtensionMessage {
+    type: "text" | "action"
+    text?: string
+    action?: "plusButtonTapped" | "settingsButtonTapped"
+}