HPCC-8030 Java/Python/Javascript language support in ECL

Added support in code generator for pluggable external language support.

New syntax: EMBED(module, 'script'), or EMBED(module) script ENDEMBED

A module for embedding Javascript using the v8 engine is provided in the plugin
v8embed - this can be imported using IMPORT javascript;

Additional plugins to support embedding Python and possibly Java will follow.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>