Improve file detection with signature check capabilities

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

runtime/syntax/javascript.yaml

@@ -2,7 +2,7 @@ filetype: javascript
 
 detect:
     filename: "(\\.js$|\\.es[5678]?$|\\.mjs$)"
-    header: "^#!.*/(env +)?node( |$)"
+    signature: "^#!.*/(env +)?node( |$)"
 
 rules:
     - constant.number: "\\b[-+]?([1-9][0-9]*|0[0-7]*|0x[0-9a-fA-F]+)([uU][lL]?|[lL][uU]?)?\\b"