Add `PD_BIN_DIR` for Consistent Variable Naming

[charlesneimog]

Introduce a new variable, PD_BIN_DIR, aligned with existing naming conventions such as PD_SOURCES_DIR, while keeping PDBINDIR for backward compatibility by emitting a deprecation warning when it is set and internally forwarding its value to PD_BIN_DIR.

For additional context, see the discussion in #22.