std: folders sorted in among the modules, without a slash

(std stm), (std test) and (std mutable) are listed where their titles
put them, with (folders mixed) and (folder-marker? #f) from glasnost
0.3.12. The mutable folder is titled (std mutable), as the others are,
so that it sorts among them.

🤖 Generated with [ECA](https://eca.dev)

Co-Authored-By: eca-agent <git@eca.dev>

modules/std/index.sz

@@ -1,2 +1,4 @@
 (meta (title "std: the standard library")
-      (sort title))
+      (sort title)
+      (folders mixed)
+      (folder-marker? #f))

modules/std/mutable/index.sz

@@ -1,2 +1,2 @@
-(meta (title "std mutable: collections that change in place")
+(meta (title "(std mutable)")
       (sort title))