feat: expose various emit functions to subclasses

[jhosteny]

Description

This change makes the various Elixir emit functions protected instead of private, so that they can be overridden in customized code generators.

Related Issue

https://github.com/glideapps/quicktype/issues/2657

Motivation and Context

Allow these functions to be overridden in subclasses.

Previous Behaviour / Output

N/A

New Behaviour / Output

N/A

How Has This Been Tested?

FIXTURE=Elixir npm test

Screenshots (if appropriate):

N/A