[Shell] Rework the Shell route parsing code so it performs better

[PureWeen]

Currently the code that parses a shell route does a lot of conversion between URI and Strings and string parsing. This is causing a measurable impact on startup performance when using even the most basic Shell App.

[jonathanpeppers]

Here is a speedscope of the Podcast app:

dotnet-podcasts.zip

If you look for MobileShell.InitializeComponent() some of the methods are:

And also:

(so some of the StartsWith calls are culture aware)

I think this is the methods in this class:

https://github.com/dotnet/maui/blob/main/src/Controls/src/Core/Shell/ShellUriHandler.cs

If this code could create one Uri per call or none, and not string.Split() and string.Join() as much, we'd be there.

[jonathanpeppers]

I think some of this was improved by #4988 and #5262.

Trying latest maui/main, I see more like:

Speedscope files: dotnet-podcasts.zip

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.