Turn IResultCapture::{push,pop}*Message into free functions

The thread-safety changes in assertions & messages turned the message
stacks into thread-local state, instead of it being member of `RunContext`.
The relevant {push,pop}(Un)ScopedMessage functions were turned from member
functions into static functions, but this left `catch_message.hpp` with
dependency on `IResultCapture`'s definition for the functions, and thus
it still had to include `catch_interfaces_capture.hpp`.

With this change, `catch_message.hpp` no longer needs
`catch_interfaces_capture.hpp`.

src/catch2/internal/catch_run_context.cpp

@@ -308,6 +308,22 @@ namespace Catch {
         }
         CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION
 
+
+        void pushScopedMessage( MessageInfo&& message ) {
+            Detail::g_messageHolder().addScopedMessage(  CATCH_MOVE( message ) );
+        }
+
+        void popScopedMessage( unsigned int messageId ) {
+            Detail::g_messageHolder().removeMessage( messageId );
+        }
+
+        void emplaceUnscopedMessage( MessageBuilder&& builder ) {
+            Detail::g_messageHolder().addUnscopedMessage( CATCH_MOVE( builder ) );
+        }
+
+        void addUnscopedMessage( MessageInfo&& message ) {
+            Detail::g_messageHolder().addUnscopedMessage( CATCH_MOVE( message ) );
+        }
     } // namespace Detail
 
     RunContext::RunContext(IConfig const* _config, IEventListenerPtr&& reporter)
@@ -937,21 +953,6 @@ namespace Catch {
         }
     }
 
-    void IResultCapture::pushScopedMessage( MessageInfo&& message ) {
-        Detail::g_messageHolder().addScopedMessage(  CATCH_MOVE( message ) );
-    }
-
-    void IResultCapture::popScopedMessage( unsigned int messageId ) {
-        Detail::g_messageHolder().removeMessage( messageId );
-    }
-
-    void IResultCapture::emplaceUnscopedMessage( MessageBuilder&& builder ) {
-        Detail::g_messageHolder().addUnscopedMessage( CATCH_MOVE( builder ) );
-    }
-
-    void IResultCapture::addUnscopedMessage( MessageInfo&& message ) {
-        Detail::g_messageHolder().addUnscopedMessage( CATCH_MOVE( message ) );
-    }
 
     void seedRng(IConfig const& config) {
         sharedRng().seed(config.rngSeed());