Add PDF rendering via PDFObject (#24086)

Use [PDFObject](https://pdfobject.com/) to embed PDFs, replacing our
outdated PDF.js copy we vendor (the last non-webpack vendoring).

[Commit
1](673e0263da)
is the PDFObject integration
[Commit
2](9336f5769d)
is the removal of PDF.js

<img width="1251" alt="Screenshot 2023-05-27 at 09 57 52"
src="https://github.com/go-gitea/gitea/assets/115237/169ce50c-bd1d-4bb0-86e5-1710bd0400a9">
<img width="1257" alt="Screenshot 2023-05-27 at 10 12 50"
src="https://github.com/go-gitea/gitea/assets/115237/318f7ee9-fb11-4093-83e7-17475aa70629">


Fallback for unsupporting browsers (most mobile ones, except Firefox
Mobile):

<img width="358" alt="Screenshot 2023-05-27 at 09 43 34"
src="https://github.com/go-gitea/gitea/assets/115237/8c12d7ba-57d6-4228-89a0-5fef9fad0cbb">

---------

Co-authored-by: Giteabot <teabot@gitea.io>

.gitattributes

@@ -2,7 +2,6 @@
 *.tmpl linguist-language=Handlebars
 /assets/*.json linguist-generated
 /public/img/svg/*.svg linguist-generated
-/public/vendor/** -text -eol linguist-vendored
 /templates/swagger/v1_json.tmpl linguist-generated
 /vendor/** -text -eol linguist-vendored
 /web_src/fomantic/build/** linguist-generated