Better context and discoverability for custom operations

[cartermp]

See here for more context: fsharp/fslang-suggestions#880

Essentially, when inside of a CE it would be nice if completion had a better understanding of the custom operations that are valid for the CE. From Saturn library:

let endpointPipe = pipeline {
    plug head
    plug requestId
}

If I type p, a valid custom operation is at the top of the completion list:

But if I type a, I get a full completion list, the vast majority of which will never actually compile:

This isn't inherently a problem, but the lack of context about the code we're in can show up pretty quickly.

Additionally, it'd be nice if there was a way to essentially query for a list of available custom operations for a given CE. This is a part of a greater set of problems around context with completions.