chore: add VS Code settings and air.toml; update .Rbuildignore

- Add .vscode/extensions.json with recommended Posit.air-vscode
- Configure formatOnSave for R and Quarto in .vscode/settings.json
- Add empty air.toml and ignore .vscode and air.toml in .Rbuildignore

Author: JamesHWade

.Rbuildignore

@@ -20,3 +20,5 @@ docs
 ^revdep$
 ^cran-comments\.md$
 ^\.claude$
+^[.]?air[.]toml$
+^\.vscode$

.vscode/extensions.json

@@ -0,0 +1,5 @@
+{
+    "recommendations": [
+        "Posit.air-vscode"
+    ]
+}

.vscode/settings.json

@@ -0,0 +1,10 @@
+{
+    "[r]": {
+        "editor.formatOnSave": true,
+        "editor.defaultFormatter": "Posit.air-vscode"
+    },
+    "[quarto]": {
+        "editor.formatOnSave": true,
+        "editor.defaultFormatter": "quarto.quarto"
+    }
+}