Improve VS Code Python settings (#2860)

chore: Fix deprecated vscode Python formatter

The python.formatting.provider setting was deprecated in favor of
having a separate extension for each Python formatter and using
editor.defaultFormatter instead. This adds a recommendation for the
Black formatter extension and selects it for Python files.

chore: Limit vscode Python analysis paths

This limits vscode's Python analysis to the scripts folders so it
doesn't slow down trying to scan all of Zephyr.

.vscode/extensions.json

@@ -1,6 +1,7 @@
 {
   "recommendations": [
     "esbenp.prettier-vscode",
+    "ms-python.black-formatter",
     "ms-python.python",
     "ms-vscode.cpptools",
     "plorefice.devicetree",