runtime(mermaid): Syntax fix (vim/vim#13774)
* runtime(mermaid): Fix arrow syntax
* runtime(mermaid): Disable syntax for identifier to avoid false match
* runtime(mermaid): Add some C++ type syntax highlight
* runtime(mermaid): Update last change time in header
a03647acc3
Co-authored-by: KSR-Yasuda <31273423+KSR-Yasuda@users.noreply.github.com>