feat: add symbolZOffset to symbol layer

[mathieupost]

Description

Add support for symbol-z-offset

I've filled the values in v8.json based on other values I saw there. I don't exactly know what each field should be (especially in expression and property-type), so I'm not 100% certain that these are correct.

Also, at some point yarn generate added the values at the right places, but for some reason it doesn't anymore for me at the moment and removes everything except for the entry in v8.json. I'm probably doing something wrong here, but I've restored what was initially added by yarn generate.

Checklist

• I've read CONTRIBUTING.md
• I updated the doc/other generated code with running yarn generate in the root folder
• I have tested the new feature on /example app.
  • Actually not in the /example app as I couldn't get it to build, but it does work in our own application FWIW (with both V11 and New Architecture enabled).
  • In V11 mode/ios
  • In New Architecture mode/ios
  • In V11 mode/android
  • In New Architecture mode/android
• I added/updated a sample - if a new feature was implemented (/example)

Screenshot OR Video

with.mov

without.mov