Always place bar in front of discriminated union

[nojaf]

I propose we add a setting to always put | before a discriminated union declaration.

The existing way of Fantomas deals with this problem is not conform to the GR style guide.
The bar gets omitted in certain scenarios. These are also, at the time of writing, not conform to what the MS style guide says.

Proposed setting name: fsharp_keep_bar_before_discriminated_union_declaration, default false.

Pros and Cons

The advantages of making this adjustment to Fantomas are correctly implementing two style guides.

The disadvantages of making this adjustment to Fantomas are one extra setting to maintain.

Examples

type A = | A of int

Extra information

Estimated cost (XS, S, M, L, XL, XXL): XS / S

Related suggestions: (put links to related suggestions here)

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• I have read the Contribution Guidelines.
• I have searched both open and closed suggestions on this site and believe this is not a duplicate

Please tick all that apply:

• This is not a breaking change to Fantomas
• I or my company would be willing to help implement and/or test this
• This suggestion is part of the Microsoft style guide : https://docs.microsoft.com/en-us/dotnet/fsharp/style-guide/formatting#formatting-discriminated-union-declarations
• This suggestion is part of the G-Research style guide :
  https://github.com/G-Research/fsharp-formatting-conventions/#formatting-discriminated-union-declarations

//cc @Smaug123