refactor(params): structure parameter IDs into typed groups

Introduce paramIds namespace with Oscillator struct and constexpr
arrays for envelope, LFO, FX, and macro parameter IDs. Replace the
scattered local const char* arrays in Engine and PluginEditor with
references to these shared definitions.

Return ModSource::Count from modSourceFromString for unknown strings
instead of silently mapping to Lfo1. Simplify isPerVoiceSource and
isPerVoiceTarget to range-based expressions.

Source/Params.cpp

@@ -50,7 +50,7 @@ ModSource modSourceFromString (const juce::String& s)
     for (int i = 0; i < kNumModSources; ++i)
         if (s == modSourceToString ((ModSource) i))
             return (ModSource) i;
-    return ModSource::Lfo1;
+    return ModSource::Count;
 }
 
 juce::String modTargetName (ModTarget t)