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.

flake.nix

@@ -32,6 +32,7 @@
             pkgs.stylua
             pkgs.selene
             pkgs.lua-language-server
+            pkgs.plantuml
           ];
         };
       });