[Feature] Status type

One of the core feature is error handling (`failed` clause). It would be nice to have named status codes. I think of it as a C# enum. Basically they are just integer constants (`Num` without fractional part).

Example:

```
// 7za https://man.archlinux.org/man/7za.1.en#DIAGNOSTICS
status L7zaStatus { // Pascal case as other Amber types
Success = 0,
Warning = 1,
Fatal = 2,
BadCommandLineParameters = 7,
NotEnoughMemory = 8,
UserCancel = 255,
}

main {
let archive = "path/to/archive.7z"
$7za x {archive}$? failed {
if {
status == L7zaStatus.UserCancel {
echo "user canceled "
}
// ... other exit codes...
else {
echo "unknown error {status}"
}
}
}
}
```

They can be defined in library (`pub` + `import`):
```
// lib.ab
pub status Foo {
...
}

// app.ab
import { Foo } from "lib.ab"
```

EDIT:

Function could somehow specify its status type:
```
fun foo_bar(a: Num, b: Num) -> Num fail L7zaStatus {
// ...
fail L7zaStatus.Fatal
}

foo_bar(1, 2) failed {
if status == L7zaStatus.Fatal { // <-- code completion in IDE provides list of L7zaStatus items
// ...
}
if status == 42 {} // <-- it is ok to use Num
}
```