Ship the revealjs filters as one Quarto extension, not vendored Lua copies #201

Found in the Quarto idiom audit.

mds, mcs, pds, sds and psw each carry root-level copies of revealjs-html-links.lua (126 lines, identical) and revealjs-level1-slide-breaks.lua (21 lines), listed under filters: by path. lds already loads the link filter as the extension d-morrison/revealjs-html-links and has its own section-slide-break.lua.

Idiomatic fix: publish both filters in one shared extension, add it with quarto add, list it by name under filters:, and delete the copies. Quarto's filter-extension docs cover the layout. Fixing a bug then happens once.

Note (tested, Quarto 1.11.9): with slide-level: 2, Quarto puts text under a # heading on the section's own slide; the break filter deliberately separates them, so keep the behavior.

Effort: small, plus one PR per site to switch over.