add a selection environement variable to access the last selection content

src/main.cc

@@ -455,6 +455,9 @@ int main(int argc, char* argv[])
     shell_manager.register_env_var("bufname",
                                    [](const Context& context)
                                    { return context.buffer().name(); });
+    shell_manager.register_env_var("selection",
+                                   [](const Context& context)
+                                   { return context.window().selections_content().back(); });
     register_commands();
     register_highlighters();
     register_filters();