Improve types for `System` module

[marmitar]

I was just verifying some types for System and both addressOf and exit seem like they have imprecise typings. I'm not sure if it is actually intended or not, so I'm opening this issue for discussion.

Return type of System.exit

System.exit is marked with void instead of never. Consider using never, like @types/node uses for process.exit.

Argument type of System.addressOf

This function is declared as addressOf(o: any): string, but any is a bit imprecise here (and almost anywhere). This function will always throw on non-object types, so the correct declaration should be addressOf(o: object): string. Keep in mind that TypeScript is aware of the difference between primitive types and their object versions (e.g. string vs String). You can test this in the REPL:

gjs> const System = imports.system
gjs> System.addressOf({ prop: 'value' })
"0xa283ae3d240"
gjs> System.addressOf(12)
typein:14:8 Error: Error invoking addressOf, at argument 0 (object): Not an object
  @typein:14:8
  @<stdin>:1:42
gjs> System.addressOf('some text')
typein:16:8 Error: Error invoking addressOf, at argument 0 (object): Not an object
  @typein:16:8
  @<stdin>:1:42
gjs> System.addressOf(new String('some text'))
"0x300cf08144f0"