Refactor frontend code

[PeterOrneholm]

Is your feature request related to a problem? Please describe.
For next major release we should have a look at how we can improve the front end related code, in which I include the HTML (Razor), JavaScript and CSS we ship in our packages. The current story to override things is to replace *.cshtml files but we could have a look if we can make things more pluggable.

What area is it related to
BankId.AspNetCore

Describe the solution you'd like
Refactor the frontend, maybe make a separate JS lib with events/hooks so consumers can override specific things.

We have some suggestions on tech improvements in #91 and #108 that's worth considering.I'll close them for now, as only making them doesn't make sense, nut rather needs to be seen as a whole.

Additional context
Will probably be a breaking change and should be planned for next major release.

I don't have a clear plan on exactly how to do this, so would like to have the conversation here before it's implemented.

[PeterOrneholm]

We should have in mind if it would be possible to write the UI as a component you can embed (and initiate) on the landing page. That would create a user interaction when you choose scheme and Android devices could automatically launch the BankID app.

[PeterOrneholm]

This was started in v6.0.0 by moving to typescript but doing a new on the client will be moved to 7..0.0