recognize xBase sources

[xBase: https://en.wikipedia.org/wiki/xBase]

Reopened PR 593 with the two language additions split off.

lib/linguist/languages.yml

@@ -1678,3 +1678,11 @@ wisp:
   ace_mode: clojure
   color: "#7582D1"
   primary_extension: .wisp
+
+xBase:
+  type: programming
+  lexer: Text only
+  color: "#3a4040"
+  primary_extension: .prg
+  extensions:
+  - .ch