EconPapers: Remove FW code; multiple enhancements

- Remove FW framework code (cf. zotero#3105)
- The code now uses Embedded Metadata; item url field's value is now
  canonical URL
- Simply use "EconPapers" as libraryCatalog value
- Detect and save more types: books and book chapters
- More metadata gathered for items: DOI, keywords, numeric keyword
  codes, original date and programming language (for software)
- Better detection of PDF links in external links; save non-PDF links
  without downloading them; do not save the (possibly many) links to
  source files for software
- Updated and added test cases, including new test cases for multiples
- Bump minVersion