fix: parsing for non-english systems, part 2

[jimnys-8]

#24 didn't fully fix the problem with parsing on non-English systems. isPrintComplete requires that both print and its own lpstat outputs English, otherwise the parsing fails and it will always return false.
In fact, execAsync always needs to output English, so instead of adding SOFTWARE= LANG=C to each command in the codebase, I rewrote execAsync so that it always adds the environment variables there. Is this an okay change, or do you plan on using execAsync for other purposes where the variables aren't needed?

PS. I changed the type definition for ExecResponse, since stdout and stderr should never be null. The code mostly ignores the possible null anyway, and npm build:declarations would throw a bunch of errors before this change.

[artiebits]

@jimnys-8 thanks for your contribution