Add luau file extension and alias to lua lexer (#1056)

Author: silverwind

lexers/embedded/lua.xml

@@ -2,8 +2,10 @@
   <config>
     <name>Lua</name>
     <alias>lua</alias>
+    <alias>luau</alias>
     <filename>*.lua</filename>
     <filename>*.wlua</filename>
+    <filename>*.luau</filename>
     <mime_type>text/x-lua</mime_type>
     <mime_type>application/x-lua</mime_type>
   </config>