Added externs.map file (replaces hard-coded list)

A new file (externs.map) has been added to the externs directory, it contains the ordered list of default externs that was previously hard-coded in the CommandLineRunner class.

The map file is not included in the externs.zip file it is packed into the compiler.jar file at the same location as the externs.zip file. At runtime the map file is read and the ordered List<String> of externs is generated.