Refactor `event_loop/threadsafe_function` file into `event_loop::js_function` module

Moves the existing threadsafe_function.rs into js_function/mod.rs, and splits initialize_source_control into its own file. Most of the noise is changing terminology from "threadsafe function" to "js function" to better clarify its purpose.

Created by  finchie  yesterday at 01:12
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