refactor(config): replace array preset syntax with preset_name = true

Problem: setup() mixed array entries (preset names) and hash entries
(custom providers keyed by filetype), requiring verbose
vim.tbl_deep_extend boilerplate to override presets.

Solution: unify under a single key=value model. Keys are preset names
or filetypes; true registers the preset as-is, a table deep-merges
with the matching preset (or registers a custom provider if no preset
matches), and false is a no-op. Array entries are dropped. Also adds
-f gfm to presets.github args so pandoc parses input as GFM.

lua/preview/presets.lua

@@ -53,11 +53,13 @@ M.github = {
   args = function(ctx)
     local output = ctx.file:gsub('%.md$', '.html')
     return {
+      '-f',
+      'gfm',
       ctx.file,
       '-s',
       '--embed-resources',
       '--css',
-      'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/github.css',
+      'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/dist/gfm.css',
       '-o',
       output,
     }