fix: support mermaid v9 by falling back to mermaid.init()

[Colty1999]

Problem

docsify-mermaid@2.0.1 calls mermaid.run() which only exists in
mermaid v10+. However, mermaid v10 requires ESM imports (type="module"),
which load asynchronously — meaning window.mermaid is undefined when the
plugin tries to use it.

Mermaid v9 loads synchronously via a regular <script> tag but only
exposes mermaid.init(), not mermaid.run().

This makes the plugin broken for both versions.

Fix

Fall back to mermaid.init() when mermaid.run() is not available,
supporting both v9 and v10+.

Changes

• src/plugin.js: check typeof mermaid.run === 'function' before
  calling it; fall back to mermaid.init() for v9
• 'README.md': updated readme to support both implementations

[Leward]

Hi @Colty1999 , thanks for the contribution. I am not watching PRs as often as I should, but I'll try to validate this and get it merged.