Refactor/preset name true syntax (#4)

* 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

* ci: remove superfluous things

* refactor: remove PreviewWatch* events and clean up docs

Problem: PreviewWatchStarted/PreviewWatchStopped were redundant with
the status() API, and the doc had a wrong author, stale INSTALLATION
format, and "watch mode" language left over from the watch → toggle
rename.

Solution: Remove the events and their tests. Fix the doc author,
rename INSTALLATION → SETUP to match sibling plugins, replace "watch
mode" with "auto-compile" throughout, and drop the events from EVENTS.

README.md

@@ -71,5 +71,3 @@ require('preview').setup({
   typst = { open = { 'sioyek', '--new-instance' } },
 })
 ```
-
-See `:h preview.nvim` for more information.