refactor(presets): simplify mermaid error parser (#49)

* feat: add `mermaid` preset

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.

* refactor(presets): simplify `mermaid` error parser

Problem: the inline `mermaid` error_parser looped over every line and
used the `Parse error on line N:` header as the message, losing the
useful `Expecting ..., got ...` token detail.

Solution: extract `parse_mermaid` alongside the other parse functions,
use a single `output:match` (mermaid's JISON parser stops at the first
error), and surface the `Expecting ..., got ...` line as the message.

* ci: format

* ci: format

flake.nix

@@ -19,7 +19,8 @@
     {
       formatter = forEachSystem (pkgs: pkgs.nixfmt-tree);
 
-      devShells = forEachSystem (pkgs:
+      devShells = forEachSystem (
+        pkgs:
         let
           devTools = [
             (pkgs.luajit.withPackages (
@@ -50,6 +51,7 @@
               pkgs.mermaid-cli
             ];
           };
-        });
+        }
+      );
     };
 }