Use the platform separator for include paths

[OskarEichler]

The -I parser hard-codes : when splitting include paths. Ruby exposes File::PATH_SEPARATOR for this purpose; on Windows the separator is ;, so a normal multi-path value remains one invalid path today.

Use the platform separator while preserving Unix behavior.

Verification:

• deterministic option-parser model fails on 2.3.1/current and passes this branch
• full suite: 76 runs / 194 assertions / 0 failures
• Ruby syntax, warning-mode load, package build and diff checks pass on Ruby 4.0.6
• combined candidate also passes 74 runs / 182 assertions on Ruby 3.2.11

No breaking behavior on platforms whose separator is :.