Warn on mixing ESM and commonJS

As part of testing for #3265 it was noticed this isn't uncommon.

As there is no way for this to be supported and it will be against how
the specification works we prefer to warn users on this at least for a
little while.

js/compiler/compiler.go

@@ -251,7 +251,15 @@ func (c *Compiler) compileImpl(
 			return nil, code, err
 		}
 		// the compatibility mode "decreases" here as we shouldn't transform twice
-		return c.compileImpl(code, filename, wrap, lib.CompatibilityModeBase, state.srcMap)
+		var prg *sobek.Program
+		prg, code, err = c.compileImpl(code, filename, wrap, lib.CompatibilityModeBase, state.srcMap)
+		if err == nil && strings.Contains(src, "module.exports") {
+			c.logger.Warningf(
+				"While compiling %q it was noticed that it mixes import/export syntax (ESM) and commonJS module.exports. "+
+					"This isn't standard behaviour and will soon not work. Please use one or the other.",
+				filename)
+		}
+		return prg, code, err
 	}
 
 	if compatibilityMode == lib.CompatibilityModeExperimentalEnhanced {

js/compiler/compiler_test.go

@@ -241,3 +241,26 @@ func TestMinimalSourceMap(t *testing.T) {
 	require.NoError(t, err)
 	require.Empty(t, hook.Drain())
 }
+
+func TestMixingImportExport(t *testing.T) {
+	t.Parallel()
+	logger := logrus.New()
+	logger.SetLevel(logrus.DebugLevel)
+	logger.Out = io.Discard
+	hook := testutils.NewLogHook(logrus.InfoLevel, logrus.WarnLevel)
+	logger.AddHook(hook)
+
+	compiler := New(logger)
+	compiler.Options = Options{
+		CompatibilityMode: lib.CompatibilityModeExtended,
+		Strict:            true,
+	}
+	_, _, err := compiler.Compile("export let s = 5;\nmodule.exports = 'something';", "somefile", false)
+	require.NoError(t, err)
+	entries := hook.Drain()
+	require.Len(t, entries, 1)
+	msg, err := entries[0].String() // we need this in order to get the field error
+	require.NoError(t, err)
+
+	require.Contains(t, msg, `it was noticed that it mixes`)
+}