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
           ];
         };
       });

lua/preview/presets.lua

@@ -302,6 +302,37 @@ M.plantuml = {
   open = true,
 }
 
+---@type preview.ProviderConfig
+M.mermaid = {
+  ft = 'mermaid',
+  cmd = { 'mmdc' },
+  args = function(ctx)
+    return { '-i', ctx.file, '-o', ctx.output }
+  end,
+  output = function(ctx)
+    return (ctx.file:gsub('%.mmd$', '.svg'))
+  end,
+  error_parser = function(output)
+    local diagnostics = {}
+    for line in output:gmatch('[^\r\n]+') do
+      local lnum = line:match('^%s*Parse error on line (%d+)')
+      if lnum then
+        table.insert(diagnostics, {
+          lnum = tonumber(lnum) - 1,
+          col = 0,
+          message = line,
+          severity = vim.diagnostic.severity.ERROR,
+        })
+      end
+    end
+    return diagnostics
+  end,
+  clean = function(ctx)
+    return { 'rm', '-f', (ctx.file:gsub('%.mmd$', '.svg')) }
+  end,
+  open = true,
+}
+
 ---@type preview.ProviderConfig
 M.quarto = {
   ft = 'quarto',