Add pl/plv8 debugging support

Feature request There (appears) to currently be no way to attach a debugger to the plv8 engine. This means that debugging/trouble-shooting is very difficult and requires a combination of console logging and modifying functions to abort & return text data early while triaging. Is your feature request related to a problem? Please describe. PostgREST allows pl/plv8 offers many interesting options, but these become increasingly more challenging to support/debug as things get more complicated. At best it's a lot of guess-work and returning text intermediate steps to try to figure out what's going on. Describe the solution you'd like My ideal would be able to attach to the v8 engine via WebStorm and/or Visual Studio Code or possibly Chrome DevTools and be able to (at a minimum) set breakpoints, inspect variables, etc. There is a V8 inspector protocol - in theory *ahem* this would involve tweaking the plv8 to ensure that the debugger is exposed and attac…