feat: variable references

[pd93]

This does the following:

• References can now be used in the base code (no experiments needed)
• If the "Map Variables" experiment (proposal 1) is enabled, references can be accessed by prefixing with #
  • This is similar to how dynamic variables are prefixed with a $.
• Adds deprecation warnings for template functions to the deprecations documentation
• Template functions can now be used in references (ref).
  • Fixes defalt and join not available in ref variables #1645
• Removes json and yaml keys from map variable experiment
  • The same functionality can now be achieved by using ref: fromJson {...}
• Docs:
  • Removed ref, yaml and json docs from Map Variables 2 experiment doc
  • Added section on ref prefix (#) in Map Variables 1 experiment
  • Added a "References" sections to the "Usage" documentation
  • Added a section on how map variables can technically be created without the map variables experiment in the variables section of the "Usage" documentation.
• Added tests for refrerences