[icon themes] Expose API to provide a dynamic icon theme.

This is an API feature request. We at `vscode-icons` extension support project detection which dynamically re-generates our icon manifest file including or not project specific icons. Additionally to that, we provide the ability to the user to provide its own custom icons and more. All these features, in the end, require the editor to be reloaded, simply because the icon manifest gets loaded at the start of the editor and with no ability to trigger a reload upon icon manifest recreation. This results in a poor UX. If we could have an API to trigger an icon manifest reload, we can have a much smoother transition between changes. Relating request: https://github.com/vscode-icons/vscode-icons/issues/1177 //cc @robertohuertasm @jens1o @PKief @EmmanuelBeziat @LaurentTreguier @file-icons @DavidBabel @be5invis