feat: Support search.min.js script customize functionality

[filzrev]

This PR intended to support search.min.js script customization.

Currently docfx modern template bundle scripts to docfx.min.js.
So it can't customize search scripts with custom template.

This PR exclude search.ts file from bundling by adding following settings

• Set modern/src/search.ts' as entryPointsto separately createsearch.min.js`.
• Rewrite ./search import statement settings by custom esbuild plugin.
  • Set external property to true to exclude from esbuild bundle
  • Rewrite path from ./search to ./search.min.js

Additionally this PR fix .NET runtime version to .NET 8 (that used by npm run start)

[yufeih]

Should we instead expose a callback method in DocfxOption for custom search implementation? That options object is the pattern for customizing JS behaviors.

[filzrev]

Should we instead expose a callback method in DocfxOption for custom search implementation? That options object is the pattern for customizing JS behaviors.

Currently DocfxOption settings are configuration is supported by main.js customization only.

On my use case. I want to switch search engine from lurn based search to PageFind based search by custom template.
In this case. user who use main.js need to copy related code from custom template/main.js.
So I hope it can be customize search by custom template.