merge: resolve conflict with origin/main

Problem: the README and vimdoc presets list omitted `plantuml` and
`mermaid` after both were added.

Solution: add both presets to the vimdoc table and the README features
blurb.

flake.nix

@@ -19,9 +19,9 @@
     {
       formatter = forEachSystem (pkgs: pkgs.nixfmt-tree);
 
-      devShells = forEachSystem (pkgs: {
+      devShells = forEachSystem (pkgs:
-        default = pkgs.mkShell {
+        let
-          packages = [
+          devTools = [
             (pkgs.luajit.withPackages (
               ps: with ps; [
                 busted
@@ -32,9 +32,24 @@
             pkgs.stylua
             pkgs.selene
             pkgs.lua-language-server
-            pkgs.plantuml
           ];
-        };
+        in
-      });
+        {
+          default = pkgs.mkShell {
+            packages = devTools;
+          };
+          presets = pkgs.mkShell {
+            packages = devTools ++ [
+              pkgs.typst
+              pkgs.texliveMedium
+              pkgs.tectonic
+              pkgs.pandoc
+              pkgs.asciidoctor
+              pkgs.quarto
+              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',