Enhancement: Quiet Output

I mostly want this for haxelib, but make it so we can have a quiet flag when compiling.
Best use for this imo is haxelib adding the flag so it doesn't spit out garbage when running an app that has warnings.

Start by tracing how compiler flags are defined and how haxelib invokes compilation. Determine whether quiet output belongs in the compiler, haxelib, or both, and identify which warnings or messages should be suppressed. Done means a documented quiet flag works for the haxelib use case without hiding required errors.

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