syntax: awk: recognize any {g,m,n,}awk script also by its shebang line

This fulfills https://savannah.gnu.org/bugs/?54516.

Suggested-by: Javier Valencia <javiervalencia80@gmail.com>

syntax/awk.nanorc

@@ -1,6 +1,7 @@
 ## Here is an example for awk.
 
 syntax awk "\.awk$"
+header "^#!.*awk"
 magic "awk script"
 comment "#"