rules: rename "tag" action to "tags"

This rule action accepts and assigns a set of 32 tags represented as a
32 bit integer just like all of river's other commands handling tags.

Using the singular here is potentially misleading and is also
inconsistent with set-view-tags, etc. which all use the plural.

Sorry about the breaking change for those who use master branch, ideally
I would have caught this before merging but at least I noticed before a
release.

This commit also does a bit of internal refactoring/cleanup of the rules
system.

river/XwaylandView.zig

@@ -285,7 +285,7 @@ fn handleSetDecorations(listener: *wl.Listener(*wlr.XwaylandSurface), _: *wlr.Xw
     const self = @fieldParentPtr(Self, "set_decorations", listener);
     const view = self.view;
 
-    const ssd = server.config.ssd_rules.match(view) orelse
+    const ssd = server.config.rules.ssd.match(view) orelse
         !self.xwayland_surface.decorations.no_border;
 
     if (view.pending.ssd != ssd) {