hide ImageType's constructor

ImageType only has static methods: a private constructor keeps it out of
the documentation and makes `new ImageType()` a type error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ljnxm1Qjrocq1izGajLx8J

Author: matmen

src/image-type.ts

@@ -25,6 +25,9 @@ function dataView(data: BinaryData): DataView {
 
 /** Detects image formats by their magic numbers */
 export class ImageType {
+    // only static methods
+    private constructor() {}
+
     /**
      * Gets an image's type from its magic number
      * @param data The image binary to get the type of