chore: prepare 2.0.0-beta.18

[skjnldsv]

Version bump and changelog for 2.0.0-beta.18:

• openers get events on a session instead of callbacks, with before-download for Text ([RFC] feat(viewer)!: tell openers with events instead of callbacks #130);
• compare(file, base, { view }), and the differences view with D (feat(viewer)!: show the differences between two files in one view #133);
• two versions of a picture over each other (feat(images): show two versions of a picture over each other #134);
• Video.js 10 instead of Plyr, with its packaged skins (feat(media): play video and audio with Video.js instead of plyr #128, refactor(media): use the packaged Video.js skins instead of a vendored copy #138);
• ESM only, and each chunk importing its own stylesheet (chore: do not inject css and build pure esm #124, fix(build): have each chunk import its stylesheet #132).

The first two and the last one break the API: the changelog has a Breaking section for them. #127 and #139 were refactor and tests only.

👾 This pull request was assisted by Claude Code, commits carry an Assisted-by trailer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.10%. Comparing base (f66759d) to head (f1ccaa3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #140   +/-   ##
=======================================
  Coverage   92.10%   92.10%           
=======================================
  Files          45       45           
  Lines        3852     3852           
  Branches      856      856           
=======================================
  Hits         3548     3548           
  Misses        283      283           
  Partials       21       21           

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