Improve file detection with signature check capabilities

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

runtime/syntax/json.yaml

@@ -2,7 +2,7 @@ filetype: json
 
 detect:
     filename: "\\.json$"
-    header: "^\\{$"
+    signature: "^\\{$"
 
 rules:
     - constant.number: "\\b[-+]?([1-9][0-9]*|0[0-7]*|0x[0-9a-fA-F]+)([uU][lL]?|[lL][uU]?)?\\b"