Process tsv-split command line arguments.

Defines the command line options, performs validation, and derives additional state. std.getopt.getopt is called to do the main option processing followed additional validation and derivation.

Help text is printed to standard output if help was requested. Error text is written to stderr if invalid input is encountered.

A tuple is returned. First value is true if command line arguments were successfully processed and execution should continue, or false if an error occurred or the user asked for help. If false, the second value is the appropriate exit code (0 or 1).

Returning true (execution continues) means args have been validated and derived values calculated. Field indices will have been converted to zero-based.

struct TsvSplitOptions
ref string[] cmdArgs