Better context and discoverability for custom operations

See here for more context: https://github.com/fsharp/fslang-suggestions/issues/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:

```fsharp
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.