Fixes: #11885

Looking at the JITStats report in PerfView (for MSBuild.exe):

| Name                                    | JitTime (ms) |
| ---                                     |          --: |
| Microsoft.Maui.Controls.Build.Tasks.dll |        214.0 |
| Mono.Cecil                              |        119.0 |

It appears that `Microsoft.Maui.Controls.Build.Tasks.dll` spends a lot
of time in the JIT. What is confusing, is this was an incremental
build where everything should already be loaded. The JIT's work should
be done already? I don't see the same behavior for Android MSBuild
tasks or Windows App SDK tasks.

The cause appears to be usage of `[LoadInSeparateAppDomain]` in .NET MAUI.

However, we can't just *remove this* as there would be complications...

`[LoadInSeparateAppDomain]` also conveniently resets all `static`
state when `<XamlCTask/>` runs again. Meaning that future incremental
builds would potentially use old (garbage) values. There are several
places that cache Mono.Cecil objects for performance reasons. Really
weird bugs would result if we didn't address this.

To make things correct *and* remove `[LoadInSeparateAppDomain]`:

* Move all `static` state to a `XamlCache` class as instance values.

* Each `ILContext` has an instance of `XamlCache`.

* Any child `ILContext`'s such as with `DataTemplate` are passed the
  parent's `XamlCache`.

* Various `static` extension methods now require a `XamlCache` to be
  passed in. This allows unit tests to do this as well.

* Other `*Visitor` or `*Converter` types have an instance of
  `ILContext`, so they can access `ILContext.Cache` as needed.

Unfortunately, I had to make small changes to lots of lines for this.

Testing these changes inside Visual Studio with a `dotnet new maui`
project template:

    Before:
    XamlCTask = 743 ms
    XamlCTask = 706 ms
    XamlCTask = 692 ms
    After:
    XamlCTask = 128 ms
    XamlCTask = 134 ms
    XamlCTask = 117 ms

This saves about ~587ms on incremental builds on all platforms, an 82%
improvement. This will help even more on large solutions with multiple
.NET MAUI projects, where `<XamlCTask/>` runs multiple times.