lint: Allow single-line braces

When using arrow functions, we only omit the braces when we are using
the return value:

    this.get_children().filter(w => w.visible);

When braces are used, eslint by default enforces line breaks, but
there are cases where the expression is hardly less concise than
the above:

    this.get_children().forEach(w => { w.destroy(); });

So change the default to allow this.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/50

lint/eslintrc-shell.json

@@ -1,6 +1,13 @@
 {
     "rules": {
         "arrow-spacing": "error",
+        "brace-style": [
+            "error",
+            "1tbs",
+            {
+                "allowSingleLine": true
+            }
+        ],
         "camelcase": [
             "error",
             {