Improvement: Update import map to repository module

[madsrasmussen]

This PR updates the import map because the module files have been moved in the frontend project.

Merge together with: umbraco/Umbraco.CMS.Backoffice#892

[bjarnef]

@madsrasmussen @bergmania I wonder if Umbraco should have a Html extension method to render this - or part of Smidge maybe? And the local ImportMapValue() can be removed.

While searching around, I found that ServiceStack has StaticImportMap and ImportMap:
https://docs.servicestack.net/javascript-add-servicestack-reference#import-maps
https://servicestack.net/posts/javascript

@Html.StaticImportMap(new() {
    ["vue"]                  = "/lib/mjs/vue.mjs",
    ["@servicestack/client"] = "/lib/mjs/servicestack-client.mjs",
    ["@servicestack/vue"]    = "/lib/mjs/servicestack-vue.mjs",
})

@Html.ImportMap(new()
{
    ["vue"]                  = ("/lib/mjs/vue.mjs",                 "/lib/mjs/vue.min.mjs"),
    ["@servicestack/client"] = ("/lib/mjs/servicestack-client.mjs", "/lib/mjs/servicestack-client.min.mjs"),
    ["@servicestack/vue"]    = ("/lib/mjs/servicestack-vue.mjs",    "/lib/mjs/servicestack-vue.min.mjs")
})

https://github.com/ServiceStack/ServiceStack/blob/3ca6daf7a2dd882437cd5cd863f22a0522eab8ec/ServiceStack/src/ServiceStack.Mvc/RazorPage.cs#L387-L408

It may also benefit developers using ES modules in frontend.

[madsrasmussen]

I really want to make it possible to add a "modules" area to the Umbraco-package.json file. In that case, it is possible for any package to expose modules to the browser and any other package will be able to use that functionality. It could be Umbraco Commerce exposing a repository as part of their package. All the core packages will be able to use the same API - the same possibilities for package developers as the core.

This will also mean that we can remove all these hardcoded entries in the razor files. How we will end up reading the Umbraco-package.json files and rendering in the template we haven't decided yet. I will leave it to the .Net experts how we best get it into the razor template. An HTML extension sounds like a good idea 👍

[iOvergaard]

Hi @bjarnef, good suggestion. Feel free to contribute with such an extension if you wish (in a new PR). I think we can put it somewhere in Umbraco.Web.Common perhaps.

Extra challenge if you are up for it: Considering that you can only have one importmap defined on the page, how would developers extend the map to append their own files?

(As Mads said, we can then use that functionality later on for some automatic parsing of manifest files to lift out global exports)