feat: use scratch buffer for file previews (#467)

* Initial implementation of scratch based preview

* Fix call to buf is valid in loop

* Fixing call to be made only from the main event loop

* Improve handling of large files from @pkazmier

* Better error handling and simplifying the code

* Default to old behavior

* Add documentation

* Fix readfile

* Fix the configuration

* refactor: single config enum and load real buffer on BufEnter

* doc: regenerate documentation

---------

Co-authored-by: Steven Arcangeli <stevearc@stevearc.com>

doc/oil.txt

@@ -157,8 +157,8 @@ CONFIG                                                                *oil-confi
       preview_win = {
         -- Whether the preview window is automatically updated when the cursor is moved
         update_on_cursor_moved = true,
-        -- Maximum file size in megabytes to preview
-        max_file_size_mb = 100,
+        -- How to open the preview window "load"|"scratch"|"fast_scratch"
+        preview_method = "fast_scratch",
         -- Window-local options to use for preview window buffers
         win_options = {},
       },