Rename ScriptRuntime to ScriptContext
SM52 essentially replaces JSRuntime with JSContext (though JSContext itself was replaced with JSCompartment).
To prepare for this migration, rename all Runtime-related things to Context.
Part of the SM52 migration, stage: SM45 compatible.
Patch by: Itms
Refs #4893
Differential Revision: https://code.wildfiregames.com/D3091