glyphs 'Replace Feature'/'Replace Prefix' custom params

We don't handle this (and I didn't know it existed.) From a cursory glance it looks like this is less for registering a feature variation than it is for just providing variable FEA? Or at least, in the case that I'm looking at (NotoSansLaoLooped) fontmake is not generating a feature variation.

Start by reproducing the NotoSansLaoLooped case and inspect how fontc handles the custom parameters “Replace Feature” and “Replace Prefix.” Compare the result with the behavior expected from the referenced fontmake output; done means these parameters are handled consistently and the feature variation behavior is covered by a regression test.

Written by the indexing model from the issue text.