feat: enable vim syntax fallback by default

Problem: languages without a treesitter parser (COBOL, Fortran, etc.)
got no syntax highlighting at all because `highlights.vim.enabled`
defaulted to `false`.

Solution: flip the default to `true`. The vim syntax path is already
deferred via `vim.schedule` so it never blocks the first paint.

lua/diffs/init.lua

@@ -124,7 +124,7 @@ local default_config = {
       max_lines = 500,
     },
     vim = {
-      enabled = false,
+      enabled = true,
       max_lines = 200,
     },
     intra = {