Add mermaid (#721)

Co-authored-by: Adam Stankiewicz <sheerun@sher.pl>

packages.yaml

@@ -5604,4 +5604,12 @@ filetypes:
 - name: openscad
   patterns:
   - pattern: '*.scad'
-    description: OpenSCAD 3D modeling language
+    description: OpenSCAD 3D modeling language
+---
+name: mermaid
+remote: mracos/mermaid.vim
+filetypes:
+- name: mermaid
+  patterns:
+  - pattern: '*.mermaid,*.mm,*.mmd'
+    description: Mermaid (https://mermaid-js.github.io/)