Describe .archify (YAML) as the diagram format everywhere (0.5.1)

- Marketplace description, README and messages lead with .archify files
  in YAML; Archify's JSON files and rendered HTML are described as also
  supported, in their own README section and a supported-files table
- Rename the rendered-HTML editor to Archify HTML Viewer (it shared the
  extension's name) and its command to Open in Archify HTML Viewer
- Empty or broken .archify files ask for valid YAML instead of JSON

src/extension.ts

@@ -35,7 +35,7 @@ export function activate(context: vscode.ExtensionContext): void {
 
   function sourceUri(arg?: unknown): vscode.Uri | undefined {
     const uri = targetUri(arg);
-    if (!uri) vscode.window.showWarningMessage('Archify: open a diagram JSON source first.');
+    if (!uri) vscode.window.showWarningMessage('Archify: open a .archify diagram first.');
     return uri;
   }