Add a Multiline option for CASE expressions (WHEN/THEN/ELSE on their own lines)

Inizia leggendo SqlScriptGeneratorOptions e il comportamento di Sql170ScriptGenerator per le opzioni Multiline* esistenti, quindi confronta il modo in cui vengono emesse le espressioni CASE. Segui i test pertinenti del generatore, se presenti, e verifica che il comportamento predefinito mantenga l'output inline, mentre MultilineCaseExpression posizioni i rami WHEN ed ELSE su righe separate e indentate.

Is your feature request related to a problem? Please describe

When Sql170ScriptGenerator regenerates a script, a CASE expression is always emitted inline — every
WHEN/THEN/ELSE is joined onto one logical line. For multi-branch CASE expressions this makes the
output substantially harder to read than the input, and where a WHEN predicate has multiple conjuncts
the hanging indent compounds: each AND is indented to the width of the preceding WHEN, and the next
WHEN is then appended after that indented continuation, producing a staircase.

Input (longest line 81 chars):

SELECT CASE
           WHEN o.status = 'P' AND o.paid_amount >= o.total_amount THEN 'settled'
           WHEN o.status = 'P' AND o.paid_amount > 0 THEN 'part-paid'
           WHEN o.status = 'C' THEN 'cancelled'
           ELSE 'open'
       END AS settlement_state,
       CASE WHEN o.due_date < SYSUTCDATETIME() THEN 1 ELSE 0 END AS is_overdue
FROM orders AS o
WHERE o.tenant_id = 42;

Output with MultilineSelectElementsList = true, IndentationSize = 4 (longest line 185 chars):

SELECT CASE WHEN o.status = 'P'
                 AND o.paid_amount >= o.total_amount THEN 'settled' WHEN o.status = 'P'
                                                                         AND o.paid_amount > 0 THEN 'part-paid' WHEN o.status = 'C' THEN 'cancelled' ELSE 'open' END AS settlement_state,
       CASE WHEN o.due_date < SYSUTCDATETIME() THEN 1 ELSE 0 END AS is_overdue
FROM   orders AS o
WHERE  o.tenant_id = 42;

An 81-character input becomes a 185-character line. There is currently no option that affects this:
of the 46 properties on SqlScriptGeneratorOptions, none matches Case or When, and none provides a
maximum line width. AlignClauseBodies, ClauseBodyAlignment and the Multiline* options do not reach
inside a CASE.

Describe the solution you'd like

A MultilineCaseExpression option (default false, preserving current output) that places each WHEN
branch and the ELSE on its own line, indented one level from the CASE:

SELECT CASE
           WHEN o.status = 'P' AND o.paid_amount >= o.total_amount THEN 'settled'
           WHEN o.status = 'P' AND o.paid_amount > 0 THEN 'part-paid'
           WHEN o.status = 'C' THEN 'cancelled'
           ELSE 'open'
       END AS settlement_state,

This fits the existing naming and behaviour of the Multiline* family —
MultilineSelectElementsList, MultilineWherePredicatesList, MultilineInValuesList,
MultilineViewColumnsList, MultilineSetClauseItems, MultilineInsertTargetsList,
MultilineInsertSourcesList, MultilineProcedureParametersList — so it should need no new concepts.

A single-branch CASE staying inline when the option is off (as
CASE WHEN … THEN 1 ELSE 0 END above) seems the right default; whether a one-branch CASE should also
break under the option is a judgement call I have no strong view on.

Describe alternatives you've considered

• Existing options — AlignClauseBodies, ClauseBodyAlignment = Indented,
  MultilineSelectElementsList, and reduced IndentationSize. None affects the interior of a CASE;
  the best combination I found did not shorten these lines.
• A general maximum-line-width option. More broadly useful, but a much larger change and a different
  model from the current per-construct toggles, so I have not requested it here.
• Post-processing the generated text. Re-wrapping CASE with a regex outside the generator defeats
  the point of using an AST-based formatter and is not reliable in the presence of string literals.

Additional context

• Microsoft.SqlServer.TransactSql.ScriptDom 180.78.1 (assembly 18.0.0.0), net8.0; behaviour is the
  same on Sql160ScriptGenerator, Sql170ScriptGenerator and Sql180ScriptGenerator.
• Related open requests of the same shape, both asking for a construct to be broken across lines:
  #189 (INSERT columns) and #21 (stored-procedure parameters).
• Happy to test a change against a large real-world T-SQL corpus if that is useful.