Add fsharp_keep_single_case_union_multiline setting

[philipcraig]

What

Adds an opt-in setting fsharp_keep_single_case_union_multiline (default false, so existing output is unchanged). When enabled, a single-case discriminated union declaration is kept on its own line instead of being collapsed onto the type name when it fits:

// default (false) — unchanged
type MyDU = Short of int

// fsharp_keep_single_case_union_multiline = true
type MyDU =
    | Short of int

Why

The goal is to let users uphold the G-Research F# formatting conventions, which list the multiline layout as the Preferred form for a single-case DU:

// Preferred
type Foo =
    | Foo of int

Today Fantomas collapses a short single-case DU onto one line — even with fsharp_bar_before_discriminated_union_declaration = true it produces type Foo = | Foo of int — so that Preferred multiline form only appears once the declaration is long enough to wrap. This setting lets people following those conventions get the Preferred shape consistently, and (independently) gives teams the uniform "one case per line regardless of case count" style, which also keeps diffs smaller when a second case is later added.

It is opt-in and off by default, so nothing changes for anyone who doesn't set it. Related prior discussion: fsharp/fslang-design#762.

How

• FormatConfig: new field KeepSingleCaseUnionMultiline (+ default). The editorconfig key is derived by reflection, so parsing/serialization are automatic.
• CodePrinter: in the single-case branch of genTypeDefn (TypeDefn.Union), use the existing multiline layout when the flag is set instead of expressionFitsOnRestOfLine. Signature files share this path.
• Docs section + CHANGELOG entry.

Tested

• New KeepSingleCaseUnionMultilineTests.fs (8 tests: fields / no-field / access modifier / bar-combination / multi-case unaffected / member case / signature file / idempotency).
• Full Fantomas.Core.Tests green (2849 passed); changed files pass fantomas --check.

Happy to bikeshed the setting name.