Typing assist: implement typing assist on delete/backspace inside records

[seclerp]

This PR implements delete/backspace assist for cases when the caret is inside record declaration/instance to help in deletion of redundant whitespace. The feature also works for anonymous records and records with attributes as well.

Examples:

Definition:

type SomeRecord =
   { field1: string
     field2: string
     {caret}field3: string }

Backspace:

type SomeRecord =
   { field1: string
     field2: string; field3: string }

Instance:

// Before
let someRecord =
   { field1 = "Quick"
     field2 = "Brown"{caret}
     field3 = "Fox" }

Delete:

let someRecord =
   { field1 = "Quick"
     field2 = "Brown"; field3 = "Fox" }

[auduchinok]

@seclerp Could you pleas add tests for cases where the typing assist shouldn't do the additional formatting, please?

An example for Delete, there's a space after the caret:

{ A = 1 {caret} 
  B = 1
  ...

Please also consider how this should work next to comments and empty lines.

[seclerp]

@auduchinok thanks for that catch, I added missing tests and also improved typing assist. Now when backspace is pressed before record field with preceding empty line (or line with only whitespace symbols) indentation is kept:

let record = {
  field1 = "Hello"

  {caret}field2 = "World"
}

Backspace:

let record = {
  field1 = "Hello"
  {caret}field2 = "World"
}