feat: add modern template customization options

[yufeih]

Enables these modern template customization options through a new public/main.js file:

• default theme
• icons next to the theme picker
• anchors
• mermaid diagrams
• syntax highlighting using highlight.js

BREAKING CHANGE: styles in modern templates are renamed to public to house scripts and styles to be published to the static sites.

fixes #8677 #8652 #8655

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (5532bf2) 76.92% compared to head (3d3b518) 76.91%.

❗ Current head 3d3b518 differs from pull request most recent head f5f999b. Consider uploading reports for the commit f5f999b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8682      +/-   ##
==========================================
- Coverage   76.92%   76.91%   -0.01%     
==========================================
  Files         606      606              
  Lines       25062    25062              
==========================================
- Hits        19278    19277       -1     
- Misses       5784     5785       +1     

see 1 file with indirect coverage changes

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[KalleOlaviNiemitalo]

Note, the breaking change also applies to custom themes, not only custom templates.

[niltor]

Is there example codes for main.js?

[yufeih]

https://github.com/dotnet/docfx/blob/main/docs/template/public/main.js and https://github.com/dotnet/docfx/blob/main/samples/seed/template/public/main.js