Slicing with builder

For now we use varLib.instancer in a bash script to slice VF with slant axis. The action is not so difficult and could be handled by the builder at the condition that the slant axis is turned into an ital axis (0:1)

The fix script should be updated to support the following missing pieces resulting in the slicing:
- [x] style linking -> already taken care by the gftools fix
- [ ] italic angle -> could be added as a new field in config.yaml
- [ ] caret run/rise -> could be calculated from UPM and italic angle

This is my current test case:
[anybody.zip](https://github.com/googlefonts/gftools/files/8142274/anybody.zip)
ps: don't mind the lasts lines in the build script: I use ttfautohint separated for now and then generate the webfonts because of the current bug.