Allow mods to write appendices

Module local values are visible to Appendixes. This mechanism can be
used for mods.

binaries/data/mods/_test.scriptinterface/module/modified/base.js

@@ -0,0 +1,4 @@
+export function fn()
+{
+	return "Base";
+}

binaries/data/mods/_test.scriptinterface/module/modified/base_not_appended.append.js

@@ -0,0 +1 @@
+throw new Error("This script should't be evaluated.");

binaries/data/mods/_test.scriptinterface/module/modified/base~0.append.js

@@ -0,0 +1,7 @@
+// Even though this is local to the module, it's visible to other appendixes.
+function appendToResult(fun, str)
+{
+	return () => fun() + str;
+}
+
+fn = appendToResult(fn, "0");

binaries/data/mods/_test.scriptinterface/module/modified/base~1.append.js

@@ -0,0 +1,4 @@
+if (import.meta.path !== "modified/base.js")
+	throw new Error("import.meta.path should be the path of the base.");
+
+fn = appendToResult(fn, "1");

binaries/data/mods/_test.scriptinterface/module/modified/base~not_appended.js

@@ -0,0 +1 @@
+throw new Error("This script should't be evaluated.");