Update Prettier settings for YAML

Editors like Zed use Prettier for YAML formatting, but this
configuration was meant for JS. GitHub Actions YAML is formatted
differently from what Prettier expects but this at least brings us a bit
closer.

This change doesn't reformat YAML files for now.

.prettierrc

@@ -5,4 +5,12 @@
 	"singleQuote": true,
 	"printWidth": 150,
 	"trailingComma": "es5",
+	"overrides": [
+		{
+			"files": "*.yml",
+			"options": {
+				"tabWidth": 2
+			}
+		},
+	]
 }