feat: Add concurrency and headless options for downloads

This commit introduces new features for controlling the download process:

-   **Concurrency**: Users can now specify the number of concurrent volume downloads using the `--concurrency` flag. This significantly speeds up the download of entire novels.
-   **Headless Mode**: A `--headless` flag has been added to control whether the browser operates in headless mode (without a visible UI). This is useful for debugging or running in environments without a display.

**Changes include:**

-   Updated `download` command to accept `--concurrency` and `--headless` flags.
-   Refactored `bilinovel` downloader to support `BilinovelNewOption` for configuring headless mode and concurrency.
-   Implemented a page pool and concurrency control mechanism within the `bilinovel` downloader to manage concurrent browser page usage.
-   Added `DownloadNovel` and `DownloadVolume` methods to the `bilinovel` downloader, utilizing goroutines and wait groups for parallel processing.
-   Updated `.vscode/launch.json` with new configurations for testing novel and volume downloads with the new options.

model/epub.go

@@ -1,6 +1,14 @@
 package model
 
-import "encoding/xml"
+import (
+	"encoding/xml"
+)
+
+type ExtraFile struct {
+	Data         []byte
+	Path         string
+	ManifestItem ManifestItem
+}
 
 type DublinCoreMetadata struct {
 	XMLName xml.Name `xml:"metadata"`