Asynchronous loading of the Viz UI

[astrojuanlu]

At the moment, Kedro-Viz works as follows:

1. kedro viz run
2. ⌛ (if the project is big, or complex etc this might take a lot of time, as it has been discussed in different issues, for example Investigate the minimum requirements for Kedro-Viz to run #1783, Kedro-viz --lite : Build DAG without importing the code #1742)
3. the UI pops up

There's another pattern that @stephkaiser and I discussed with somebody one day:

1. kedro viz run
2. ⏩ the UI launches more or less instantaneously
3. ⌛ the skeleton is there, filled with some "loading..." spinners
4. the UI gets populated

Given that we are using a SPA framework for the frontend already, we could use it to load project information asynchronously after the UI has launched, and increase the perceived performance of Kedro Viz at launch.