feat: update scripts to reflect new fixture structure

[sdoire]

This PR updates two scripts, update-fixtures.js and generate-custom-parser.js, to take into account the new fixture structure implemented in #713. @johnholdun was responsible for most of the code changes -- I only had to tweak a couple of things to make it work from the original changes (such as removing the creation of a fixtures folder in this commit). See video below for demo of the generate-custom-parser.js script working to correctly create the new custom parser files and fixture in the correct location.

https://user-images.githubusercontent.com/10479709/226418070-52eefd09-b270-4485-acab-81b31a8b631f.mov