UserspaceEmulator: Enable splitting regions at arbitrary points

This is not yet useful in and of itself, but enables the feature in the next commit.

Userland/DevTools/UserspaceEmulator/Region.h

@@ -81,6 +81,7 @@ public:
 
 protected:
     Region(u32 base, u32 size);
+    void set_range(Range r) { m_range = r; };
 
 private:
     Emulator& m_emulator;