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.

README.md

@@ -55,11 +55,8 @@ require('preview').setup({
 **Q: How do I override a preset?**
 
 ```lua
-local presets = require('preview.presets')
 require('preview').setup({
-  typst = vim.tbl_deep_extend('force', presets.typst, {
-    env = { TYPST_FONT_PATHS = '/usr/share/fonts' },
-  }),
+  typst = { env = { TYPST_FONT_PATHS = '/usr/share/fonts' } },
 })
 ```
 
@@ -70,8 +67,8 @@ open the output with `vim.ui.open()` after the first successful compilation.
 For a specific application, pass a command table:
 
 ```lua
-typst = vim.tbl_deep_extend('force', presets.typst, {
-  open = { 'sioyek', '--new-instance' },
+require('preview').setup({
+  typst = { open = { 'sioyek', '--new-instance' } },
 })
 ```