refactor(config): replace array preset syntax with preset_name = true (#3)

* 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.

* ci: format

* fix(presets): parenthesize gsub output to suppress redundant-return-value

lua/preview/init.lua

@@ -68,15 +68,17 @@ function M.setup(opts)
     if k == 'debug' then
       vim.validate('preview.setup opts.debug', v, { 'boolean', 'string' })
       debug = v
-    elseif type(k) == 'number' then
-      vim.validate('preview.setup preset name', v, 'string')
-      local preset = presets[v]
+    elseif type(k) ~= 'number' then
+      local preset = presets[k]
       if preset then
-        providers[preset.ft] = preset
+        if v == true then
+          providers[preset.ft] = preset
+        elseif type(v) == 'table' then
+          providers[preset.ft] = vim.tbl_deep_extend('force', preset, v)
+        end
+      elseif type(v) == 'table' then
+        providers[k] = v
       end
-    else
-      vim.validate('preview.setup provider config', v, 'table')
-      providers[k] = v
     end
   end