CLI: -v, and argument errors that read as errors

- `scriptc -v` / `--version` prints the installed version, read from the package manifest so a release build and a source checkout answer the same way. It previously fell through to the compile path and crashed trying to read `-v` as an entry file.
- An unknown flag or a missing flag value now prints one line naming what was wrong, followed by usage. parseArgs' ERR_PARSE_ARGS_* used to escape to the top level and paint a Node stack trace over the terminal.
- Reworded `--backend` help: llvm is the default and the output that ships, c is the readable reference backend for inspecting what the compiler produced.