Every single `TagHelperDescriptorProviderContext` created in Razor compiler or tooling code adds a valid compilation. So, adding the compilation to the `ItemsCollection` is pure overhead and no `ITagHelperDescriptorProvider` needs to check it for null.

I removed a single test that verified the behavior if a compilation was never set. Now that a compilation is required, this test is no longer needed.

This can be safely removed now that there are no more references.

- Enable nullability
- Mark all as sealed
- Use `Lazy<T>` for singleton `ITagHelperDescriptorProviders`
- Introduce `TagHelperDescriptorProviderBase`
- Inherit from `TagHelperDescriptorProviderBase`
- Remove unnecessary properties
- Remove unnecessary property setters

In addition, I spotted a bug in `EventHandlerTagDescriptorProvider` while addressing nullability warnings. The buggy code is right here:

https://github.com/dotnet/razor/blob/fb84ae5d9bb8132972c2c23cf209721161f81deb/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/CSharp/EventHandlerTagHelperDescriptorProvider.cs#L70-L76

When reading the constructor arguments of an `EventHandlerAttribute` it appears that we swap the last two arguments of the 4-argument variant. The constructor is defined like so:

```C#
EventHandlerAttribute(string attributeName, Type eventArgsType, bool enableStopPropagation, bool enablePreventDefault);
```

Unfortunately, the compiler reads the third parameter as `enablePreventDefaeult` and the fourth parameter as `enableStopPropagation`.

This has been broken since support for the 4-argument constructor variant was added in 7635bba. Fixing this bug is tracked by #10497.

* Move to central package pinning

This should make it much easier for us to respond to CG alerts in the
future. All that will need to be done is add an entry in
Directory.Packages.props and it will automatically impact all consumers
of it.

Consider this example in Roslyn for how to respond to a CG issue

dotnet/roslyn#74653

Every `TagHelperDescriptorProviderContext` creates an `ItemCollection`
to hold onto, at most, two objects: a `Compilation` and a target
`ISymbol`. This is enormously wasteful because an `ItemCollection`
internally creates a `ConcurrentDictionary<object, object>`. So, I've
changed `TagHelperDescriptorProviderContext` to just hold onto a
compilation and a target symbol and avoid the `ItemCollection`
altogether.

I recommend reviewing commit-by-commit.

Also, I bumped into a long standing compiler bug in
`EventHandlerTagHelperDescriptorProvider` that was recently filed by a
customer: #10497. I opted to stay
on target and not fix this issue, but I made it painfully obvious where
the fix would go. 😄