[wasm] Example for `FPDF_LoadCustomDocument ` method

[nguyenyou]

I'm looking at the opportunity for open document from input stream on web (open Linearized PDFs), do you think streaming PDF is possible with pdfium?

Refs:

• https://apryse.com/blog/mobile/streaming-a-pdf-from-the-web
• https://www.nutrient.io/guides/web/performance/streaming/