NETSCRIPT: Add Experience gain to formula.gang

[DominicDams]

Description

When trying to automate gang activities I realized that there isn't currently an API for gainExperience that is exposed in Netscript. Adding an API for this should simply involve moving code over from https://github.com/bitburner-official/bitburner-src/blob/dev/src/Gang/GangMember.ts#L156-L193 to https://github.com/bitburner-official/bitburner-src/blob/dev/src/Gang/formulas/formulas.ts and then updating the documentation.

Ownership

I'd be happy to contribute a code change for this in a couple of days, just wanted to make sure it would be accepted before I bothered with it. If you don't think it's a worthwhile enhancement/has some unforeseen footgun let me know and I can drop this (I'll likely write the code on my end to calculate this in Netscript for the time being either way)

[d0sboots]

I think having more formulas for Gang would be great! One tricky thing is that there might be some new objects that you need to introduce publicly to make such an API work.