Consider having different builds for Zotero 7 and Zotero 6/web library?

[tnajdek]

Reader is currently building pdf.js in "generic-legacy" mode and is set for babel compatibility with FF60. Both these things are needed for Zotero 6 but for Zotero 7 we can probably do away without most of it? This could even, potentially, maybe, slightly improve performance in Zotero 7?

Also this makes even more sense given that in https://github.com/zotero/reader/pull/125 I've introduced even more compatibility tweaks (web-library is targeting Safari 11 compatibility)