(maint) Enable the code spell extension for MDX

By default the MDX files are not spell checked with code spell checker.
This updates the settings to enable spell check of these files.

Author: corbob

.vscode/settings.json

@@ -92,6 +92,9 @@
         "windirstat",
         "xcopy"
     ],
+    "cSpell.enableFiletypes": [
+        "mdx"
+    ],
     "languageToolLinter.languageTool.ignoredWordsInWorkspace": [
         "format-filesize",
         "get-packageparameters",