feat: add mermaid preset (#46)

Problem: no built-in support for compiling mermaid diagrams via `mmdc`.

Solution: add a `mermaid` preset that compiles `.mmd` files to SVG and
parses `Parse error on line N` diagnostics from stderr. Add
`mermaid-cli` to the nix dev shell.

flake.nix

@@ -33,6 +33,7 @@
             pkgs.selene
             pkgs.lua-language-server
             pkgs.plantuml
+            pkgs.mermaid-cli
           ];
         };
       });