Add a whenHeld to a custom trigger class

[TechplexEngineer]

If/When they merge wpilibsuite/allwpilib#1643 we can remove this shim

[derrell]

I'm not so sure about this. I think it's a bad idea to be mixing use of BionicTrigger and Trigger. We should use native stuff, or use our own wrapper, but not both. How about adding the required functionality included here into BionicTrigger?

[roshanr10]

if we're going to be doing this, why don't we inherit BionicTrigger from Trigger so they're more interchangeable? if your PR goes through, then it'll become native and no changes to be made

[TechplexEngineer]

@roshanr10 It would be good for someone to try to extend Trigger, but I was running into access issues. Please do try.