[API Proposal]: Align Await implementation

[gusty]

Background and motivation

dotnet/fsharp#19785 (comment)

API Proposal

See linked discussion

API Usage

See linked discussion

Alternative Designs

See linked discussion

Risks

No response

Are you willing to help with a proof-of-concept (as PR in that or a separate repo) first and as pull-request later on?

No

[bartelink]

.@gusty Sorry for the [att|nagg]ing but if Tomas's PR can be reviewed and/or merged, it would unblock getting Await (inc the tasklike support) into FSharp.Core main (which also brings bartelink/fsharp#1 addressing fsharp/fslang-suggestions#1284 into play)