System.Linq.Expressions currently offers multiple build time definitions to build different flavors of the library:

* `FEATURE_COMPILE` (defined everywhere except iOS/tvOS/Catalyst, and NativeAOT) - enables Reflection.Emit-based expression tree compiler.
* `FEATURE_INTERPRET` (always defined and not actually possible to build without) - enables the expression interpreter
* `FEATURE_DLG_INVOKE` (defined everywhere except NativeAOT, but we likely need to be able to run without it on iOS too because there's uninvestigated bugs around it ActiveIssue'd in dotnet#54970) - in the interpreter, use a delegate to invoke `CallInstructions` instead of `MethodInfo.Invoke`. The delegate might have to be Reflection.Emitted if it's not supportable with `Func`/`Action` (that's the uninvestigated iOS/tvOS/Catalyst bug).

For dotnet#61231, we need to be able to build a single System.Linq.Expression library that can switch between these build-time configurations _at publish time_ since we don't want to build a separate S.L.Expressions library for NativeAOT. There are advantages in having this setup for non-NativeAOT scenarios too. This pull request accomplishes that by mechanically changing the `#define`s into feature switches.

The feature switch is placed in the last proposed location for dotnet#17973. I expect we'll want such API to be public at some point; now that Xamarin and NativeAOT use this formerly .NET Native-only thing the API request became relevant again.

This pull request is focused on the mechanical replacement of `#defines` with feature switches and it's already a lot bigger than I'm comfortable with.

There's some obvious "`!FEATURE_COMPILE` means this is .NET Native with everything what that meant" that I did not touch because this is meant to be a mechanical change. Some cleanup will be needed at some point. Right now this just mostly means we're running fewer tests than we could.

Validation:
* Verified that we're still running the same number of tests with CoreCLR as we previously were and they're all passing.
* Verified we're getting mostly the same size of the S.L.Expressions library on iOS (433 kB grew to 437 kB, the diffs are expected).
* Verified things work on the NativeAOT side as well.