(regression) Fix lua_requireinternal to use variable number of stack args (luau-lang#1804)

Since 0.672 the `lua_proxyrequire` function is broken and attempt to use
it fails with the error message `module must return a single value`.

This function takes two stack arguments (`lua_require` takes one), but
`lua_requireinternal` expects only one extra argument and pushes another
one (`cacheKey`).
As a result, the continuation function `lua_requirecont` (that has
hardcoded number of stack args = 2) thinks that module returned more
than one result and throw an exception.

The fix instead of hardcoding number of arguments in the continuation
function, pass this number in a first stack argument.
This will allow both `lua_require` and `lua_proxyrequire` work at the
same time.

Author: khvzak

Require/Runtime/src/RequireImpl.cpp

@@ -121,9 +121,9 @@ static int checkRegisteredModules(lua_State* L, const char* path)
 int lua_requirecont(lua_State* L, int status)
 {
     // Number of stack arguments present before this continuation is called.
-    const int numStackArgs = 2;
+    const int numStackArgs = lua_tointeger(L, 1);
     const int numResults = lua_gettop(L) - numStackArgs;
-    const char* cacheKey = luaL_checkstring(L, 2);
+    const char* cacheKey = luaL_checkstring(L, numStackArgs);
 
     if (numResults > 1)
         luaL_error(L, "module must return a single value");
@@ -152,6 +152,8 @@ int lua_requirecont(lua_State* L, int status)
 
 int lua_requireinternal(lua_State* L, const char* requirerChunkname)
 {
+    int stackTop = lua_gettop(L);
+
     // If modifying the state of the stack, please update numStackArgs in the
     // lua_requirecont continuation function.
 
@@ -171,10 +173,14 @@ int lua_requireinternal(lua_State* L, const char* requirerChunkname)
     if (resolvedRequire.status == ResolvedRequire::Status::Cached)
         return 1;
 
-    // (1) path, (2) cacheKey
+    // (1) path, ..., cacheKey
     lua_pushstring(L, resolvedRequire.cacheKey.c_str());
 
-    int numArgsBeforeLoad = lua_gettop(L);
+    // Insert number of arguments before the continuation to check the results.
+    int numArgsBeforeLoad = stackTop + 2;
+    lua_pushinteger(L, numArgsBeforeLoad);
+    lua_insert(L, 1);
+
     int numResults = lrc->load(L, ctx, path, resolvedRequire.chunkname.c_str(), resolvedRequire.contents.c_str());
     if (numResults == -1)
     {