[Blazor] Compute relative root in preloading paths

[maraf]

We are using relative paths in preloading headers. For routes containing several segments, like /path/to/page, we need to make preloading paths relative to the root, as ../../_framework/dotnet.js. The reason why absolute paths might be is that the whole blazor app can be nested in some subpath.

• Blazor hosted in root /
  • @page "/counter"
    • url in browser /counter
    • preloading path _framework/dotnet.js
  • @page "/folder/path"
    • url in browser /folder/path
    • preloading path ../_framework/dotnet.js
  • @page "/folder1/folder2/path"
    • url in browser /folder1/folder2/path
    • preloading path ../../_framework/dotnet.js
• Blazor hosted on /subpath
  • @page "/counter"
    • url in browser /subpath/counter
    • preloading path _framework/dotnet.js
  • @page "/folder/path"
    • url in browser /subpath/folder/path
    • preloading path ../_framework/dotnet.js
  • @page "/folder1/folder2/path"
    • url in browser /subpath/folder1/folder2/path
    • preloading path ../../_framework/dotnet.js

A problematic bit is if user adds a trailing slash to the URL, like /counter/

Fixes #62174