Add MultilineCaseExpression script generator option

[trg-alasdair]

Description

Fixes #226.

SqlScriptGenerator always writes a CASE expression on one line. A multi-branch CASE becomes one long line, and when a WHEN predicate contains AND, MultilineWherePredicatesList breaks it and appends the next WHEN after the continuation, which produces the staircase shown in the issue.

This adds MultilineCaseExpression (default false). When it is true, each WHEN clause and the ELSE are written on their own line, indented one level from CASE, and END is written on its own line aligned with CASE:

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

• It applies to simple and searched CASE, wherever the expression appears. A nested CASE aligns with its own CASE keyword.
• The input expression of a simple CASE stays on the CASE line.
• WHEN predicates keep their existing behavior under MultilineWherePredicatesList. Because each WHEN now starts its own line, the continuations no longer compound.
• A trailing single-line comment on a branch stays on that branch's line. Today it moves to the end of the statement, because the deferred comment is written out at the next line break.
• With the option off, the output is unchanged: the alignment point is pushed only when the option is on.

The setting is in SqlScriptGeneratorOptions.xml in a new "CASE expressions" group, and the generator change is confined to SqlScriptGeneratorVisitor.CaseExpression.cs.

This change was written with the help of an AI coding assistant, and I reviewed and tested it.

Code Changes

• Unit tests are added: CaseExpressionFormattingTests (12 tests), covering the default, simple and searched CASE, no ELSE, a single branch, nesting, COLLATE, IndentationSize, a trailing comment, and the example from Add a Multiline option for CASE expressions (WHEN/THEN/ELSE on their own lines) #226 with otherwise-default options
• Existing tests are passing: all 1147 tests in UTSqlScriptDom on net8.0 (macOS)
• New or updated code follows the guidelines here

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