Visual: toggle page light-dark theme

[cpelley]

• ENH: Page theme toggle.

System dark theme	System light theme	System dark, light override using toggle*1

*1 Note how the mermaid diagram does not as of yet change theme with toggle.
The same would be true of a system setup to have a light theme, then clicking the theme toggle override to dark.
mermaid graph re-initialisation isn't trivial (mermaid doesn't support this) - for now we issue an alert to say that this isn't yet supported. I will introduce the community workaround to achieve this in a future PR.

• ENH: Mermaid diagram theme set on initialisation, respecting system theme, whether light or dark.
• ENH: Support initialising mermaid on user behalf if not done so already. This means we can dispense from including mermaid initialisation from the user html file (though this is still supported).
• BUG: Fixed subgraph ID nesting deduced from '_' - incorrect accounting of subgraph closures (encountered in testing by @SamGriffithsMO).
• BUG: Missing table text wrap toggle title.
• Table now sortable by clicking on a column header (toggling between ascending and descending order based on this clicked table column header).
  e.g. order table by descending order or the second row:

• Table item delimiter now toggleable between <br> (newline) and ; .
  
  

Background

• https://web.dev/articles/light-dark ("CSS color-scheme-dependent colors with light-dark()").
• https://www.mostlylucid.net/blog/switchingthemesformermaid (mermaid diagram re-initialisation).

Issues

• Closes Add dark-mode toggle to DAG visualisation #77
• https://github.com/MetOffice/improver_suite/issues/2079

[cpelley]

The visualisation isn't operational and requires a fast iterative development process to reach our goals.
To that end, I'm going to proceed with its merge. Do feel free to post feedback in an issue and I'll address on the next iteration.