Improve file detection with signature check capabilities

This allows more complex detection upon regex rules for a certain amount of
lines.

runtime/syntax/csx.yaml

@@ -1,7 +1,7 @@
 filetype: csharp-script
 detect:
     filename: "\\.csx$"
-    header: "^#!.*/(env +)?dotnet-script( |$)"
+    signature: "^#!.*/(env +)?dotnet-script( |$)"
     
 rules:
     - include: "csharp"