nominate tmandry as co-lead of the language design team

The lang team recently held its process to select the new co-lead, using the same process proposed by the compiler team. The result is that @tmandry has been nominated to serve as the new co-lead. To make it official, I'm going to conduct a T-lang FCP.

Author: nikomatsakis

teams/lang.toml

@@ -1,7 +1,10 @@
 name = "lang"
 
 [people]
-leads = ["nikomatsakis"]
+leads = [
+    "nikomatsakis",
+    "tmandry",
+]
 members = [
     "joshtriplett",
     "nikomatsakis",