Expose ofAsyncEnum/toAsyncEnum on netstandard2.0

The .fsi signature file was gating ofAsyncEnum, toAsyncEnum, and
ofIQueryable behind NETSTANDARD2_1 || NETCOREAPP3_0, while the .fs
implementation already guarded them with NETSTANDARD || NET (which
includes netstandard2.0). Since the project already depends on
Microsoft.Bcl.AsyncInterfaces which provides IAsyncEnumerable<T> for
netstandard2.0, aligning the .fsi guard makes these functions available
on that target framework.

Closes #173

Co-authored-by: Copilot <[email protected]>

Author: Copilot

src/FSharp.Control.AsyncSeq/AsyncSeq.fsi

@@ -549,7 +549,7 @@ module AsyncSeq =
     /// completion of sub-sequences depends on completion of other sub-sequences.
     val groupBy<'T, 'Key when 'Key : equality> : projection:('T -> 'Key) -> source:AsyncSeq<'T> -> AsyncSeq<'Key * AsyncSeq<'T>>
 
-    #if (NETSTANDARD2_1 || NETCOREAPP3_0)
+    #if (NETSTANDARD || NET)
 
     /// Creates an asynchronous computation that asynchronously yields results from the provided .NET IAsyncEnumerable.
     val ofAsyncEnum<'T> : source: Collections.Generic.IAsyncEnumerable<'T> -> AsyncSeq<'T>