feat: add plantuml preset (#45)

Problem: PlantUML (`.puml`) diagrams have no built-in preview support,
and Neovim lacks filetype detection for PlantUML files.

Solution: Add a `plantuml` preset that compiles to SVG via `plantuml
-tsvg`, with an error parser for `Error line N` diagnostics. Register
`.puml` and `.pu` extensions via `vim.filetype.add` when the preset is
configured. Add `plantuml` to the nix dev shell.

lua/preview/init.lua

@@ -105,6 +105,12 @@ function M.setup(opts)
     vim.validate(prefix .. '.detach', provider.detach, 'boolean', true)
   end
 
+  if providers['plantuml'] then
+    vim.filetype.add({
+      extension = { puml = 'plantuml', pu = 'plantuml' },
+    })
+  end
+
   config = vim.tbl_deep_extend('force', default_config, {
     debug = debug,
     providers = providers,