Add editor, ignore, and prettier configuration

package.json

@@ -0,0 +1,8 @@
+{
+    "name": "x",
+    "private": true,
+    "description": "prettier is used by `make fmt` and `make fmt-check` to format markdown",
+    "devDependencies": {
+        "prettier": "3.9.10"
+    }
+}