Various Mermaid improvements (#18776) (#18780)

* Various Mermaid improvments

- Render into iframe for improved security
- Use built-in dark theme instead of color inversion
- Remove flexbox attributes, resulting in more consistent size rendering
- Update API usage and update to latest version

* restart ci

* misc tweaks

* remove unneccesary declaration

* make it work without allow-same-origin, add loading=lazy

* remove loading attribute, does not seem to work

* rename variable

* skip roundtrip to DOM for rendering

* don't guess chart height

* update comment to make it clear it's intentional

* tweak

* replace deprecated 'scrolling' property

* remove unused css file

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/index.less

@@ -10,7 +10,6 @@
 @import "./features/codeeditor.less";
 @import "./features/projects.less";
 @import "./markup/content.less";
-@import "./markup/mermaid.less";
 @import "./markup/codecopy.less";
 @import "./code/linebutton.less";