Add Swift shebang to syntax (zyedidia#3451)

The Swift compiler can be run in "interpreter" mode, so it can run Swift "scripts" if they have a proper shebang and no file extension.

Author: frkd-dev

runtime/syntax/swift.yaml

@@ -2,6 +2,7 @@ filetype: swift
 
 detect:
     filename: "\\.swift$"
+    header: "^#!.*bin/(env +)?swift( |$)"
 
 rules: