v0.1.21 of @opendocsg/pdf2md uses the latest version of pdfjs-dist
and adjusts its codebase to lineup with changes to the PDF.js API.
Drop this into obsidian-extract-pdf so that the project no longer
needs to maintain its own fork of pdf2md.

- Consolidate pdf2md dependencies to the official one
- Explicitly bring in pdfjs-dist
- Change codebase so that we just call pdf2md to get the Markdown text,
  rather than copy its implementation
- Adjust module declarations to pick up the new dependencies,relying
  on TypeScript's module resolution to look inside node_modules
- Remove patch-package, no longer needed