add missing extension for C3 language

Author: thechampagne

lib/linguist/languages.yml

@@ -886,6 +886,7 @@ C3:
   color: "#2563eb"
   extensions:
   - ".c3"
+  - ".c3i"
   tm_scope: source.c3
   ace_mode: c_cpp
   codemirror_mode: clike