getInt method
Load the enviroment variable value as an int
If variable with name does not exist then fallback will be used.
However if also no fallback is supplied an error will occur.
Furthermore an FormatException will be thrown if the variable with name
exists but can not be parsed as an int.
Implementation
int getInt(String name, {int? fallback}) {
final value = maybeGet(name);
assert(value != null || fallback != null,
'A non-null fallback is required for missing entries');
return value != null ? int.parse(value) : fallback!;
}