Show CSS grid line numbers (#235)

To allow to better design and debug  CSS grids [1 the Firefox "DevTools"
can visually show the grid line numbers [2].
This option is disabled by default and is now enabled by default.

[1]: https://css-tricks.com/snippets/css/complete-guide-grid
[2]: https://developer.mozilla.org/en-US/docs/Tools/Page_Inspector/How_to/Examine_grid_layouts#Display_line_numbers

Closes GH-223

snowblocks/firefox/user.js

@@ -302,6 +302,7 @@ user_pref("devtools.editor.detectindentation", true);
 user_pref("devtools.editor.expandtab", true);
 user_pref("devtools.editor.keymap", "default");
 user_pref("devtools.editor.tabsize", 2);
+user_pref("devtools.gridinspector.showGridLineNumbers", true);
 user_pref("devtools.responsive.reloadConditions.touchSimulation", false);
 user_pref("devtools.responsive.reloadConditions.userAgent", false);
 user_pref("devtools.responsive.showUserAgentInput", false);