fix: stack overflow in table.concat function

The previous implementation of tableConcat was pushing all values and
separators onto the Lua stack before concatenating them, which caused
stack overflow with large tables.

This fix changes the implementation to build the result string
incrementally rather than pushing all values onto the stack first,
preventing stack overflow with large tables.

Example that would previously fail but now works correctly:
```
package main

import "github.com/yuin/gopher-lua"

func main() {
	var L *lua.LState
	var err error

	L = lua.NewState()
	defer L.Close()

	err = L.DoString(`
		local t = {}
		for i = 1, 10000 do
			t[i] = tostring(i)
		end
		local s = table.concat(t, ',')
	`)
	if err != nil {
		println(err.Error())
	}
}
```

Author: thierry-f-78

tablelib.go

@@ -66,19 +66,18 @@ func tableConcat(L *LState) int {
 		L.Push(emptyLString)
 		return 1
 	}
-	//TODO should flushing?
-	retbottom := L.GetTop()
+	result := ""
 	for ; i <= j; i++ {
 		v := tbl.RawGetInt(i)
 		if !LVCanConvToString(v) {
 			L.RaiseError("invalid value (%s) at index %d in table for concat", v.Type().String(), i)
 		}
-		L.Push(v)
+		result += LVAsString(v)
 		if i != j {
-			L.Push(sep)
+			result += LVAsString(sep)
 		}
 	}
-	L.Push(stringConcat(L, L.GetTop()-retbottom, L.reg.Top()-1))
+	L.Push(LString(result))
 	return 1
 }