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

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "archify-vscode-ext",
-  "version": "0.5.0",
+  "version": "0.5.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "archify-vscode-ext",
-      "version": "0.5.0",
+      "version": "0.5.1",
       "license": "MIT",
       "dependencies": {
         "yaml": "^2.9.1"