Add .app.src as Erlang extension. (#2964)

* Add .app.src as Erlang extension.

* Add .app.src sample.

lib/linguist/languages.yml

@@ -948,6 +948,7 @@ Erlang:
   color: "#B83998"
   extensions:
   - .erl
+  - .app.src
   - .es
   - .escript
   - .hrl

samples/Erlang/sample.app.src

@@ -0,0 +1,8 @@
+{application, sample,
+ [{description, "sample app"},
+  {vsn, "1.0.0"},
+  {registered, []},
+  {mod, {sample_app, []}},
+  {applications, [kernel, stdlib]},
+  {env, []},
+  {modules, []}]}.