Simple backend-independent plotting for PDL

Changes for 1.011 - 2024-04-22

• P{GPLOT,Lplot} to read devices using proper API not subprocesses
• if PDL_SIMPLE_ENGINE in ENV, use ONLY that if engine unspecified
• each driver module now publishes which $P:G:S::API_VERSION it conforms to
• PGS::register takes hashref instead of magic var in driver namespace
• loading driver module not eval-ed
• PDL_SIMPLE_DEVICE in env replaces driver guessing
• PDL_SIMPLE_OUTPUT in env overrides input parameters