src: Add a window_range default expansion

This commit adds a `window_range` default expansion that holds the
coordinates and size of the buffer-space on the window.

Fixes #675

src/main.cc

@@ -280,6 +280,17 @@ static const EnvVarDesc builtin_env_vars[] = { {
         "user_modes", false,
         [](StringView name, const Context& context, Quoting quoting) -> String
         { return join(context.keymaps().user_modes(), ' ', false); }
+    }, {
+        "window_range", false,
+        [](StringView name, const Context& context, Quoting quoting) -> String
+        {
+            const auto top_left = context.window().display_position({0, 0});
+            const auto window_dim = context.window().dimensions();
+
+            return format("{} {} {} {}", top_left->line, top_left->column,
+                                         window_dim.line - top_left->line,
+                                         window_dim.column - top_left->column);
+        }
     }
 };