Add a right-aligned clause keyword ("river") value to ClauseBodyAlignment

[kspeeckaert]

Feature request: right-aligned clause keywords ("river" style) as a ClauseBodyAlignment option

Is your feature request related to a problem? Please describe.

ClauseBodyAlignment currently offers Aligned and Indented. Aligned pads to the right of the clause keyword, so keywords are flush left and the clause bodies line up:

SELECT   a,
         COUNT(*)
FROM     t
GROUP BY a
HAVING   COUNT(*) > 1
ORDER BY a;

There is no way to get the widely used variant where the keywords themselves are right-aligned into the gutter:

  SELECT a,
         COUNT(*)
    FROM t
GROUP BY a
  HAVING COUNT(*) > 1
ORDER BY a;

This is a common house style, and for anyone who wants it, ScriptDOM's script generator currently can't produce it at all — not via any combination of the existing options.

Describe the solution you'd like

A third value on the existing ClauseBodyAlignment enum, for example KeywordRightAligned (or River, if you prefer the common name for the style), leaving Aligned as the default so no existing output changes.

The reason this is likely a small change: both styles place the clause body in exactly the same column. In the example above, the body column is 9 in both cases, derived from the width of the widest clause keyword in the statement (GROUP BY). The gutter arithmetic that Aligned already performs is exactly what the new value needs — the only difference is whether the padding is emitted after the keyword or before it.

Concretely, the affected pieces look like:

• SqlScriptDom/ScriptDom/SqlServer/Settings/SqlScriptGeneratorOptions.xml — one new <Option> entry on the existing setting
• SqlScriptDom/ScriptDom/SqlServer/ClauseBodyAlignment.cs — one new enum member
• SqlScriptDom/ScriptDom/SqlServer/ScriptGenerator/ScriptWriter.cs — emit the alignment padding before the keyword rather than after it

There already exists a RightAlignedSeparatorElement in ScriptWriter, used to right-align leading commas so they end at the following alignment point. That is close to the same operation applied to a clause keyword, so the concept is arguably already present in the writer.

For nested constructs, the natural behaviour would be for the gutter to be computed per statement at each nesting level, matching how Aligned already scopes its alignment points.

Describe alternatives you've considered

• Post-processing the generated script. Workable in principle, since the body column is already correct: move each clause keyword's trailing padding to the front of the line. In practice it means re-deriving line structure from text, taking care not to touch lines inside multi-line string literals or block comments, and handling the existing indent of nested subqueries. That is a fragile reimplementation of information the writer already has.
• Post-processing GenerateTokens() output. More robust than working on text, but still amounts to redoing the alignment pass outside the library.
• Indented mode. Different style, doesn't address this.

Additional context

Prior art for this as a first-class formatter option:

• Oracle SQL Developer added a "Right-Align Query Keywords" formatting option in 18.1
• DataGrip has a long-standing request for the same (DBE-5419)
• Several standalone formatters implement it as their primary style

I'm using ScriptDOM directly for a small standalone T-SQL formatter, driven by SqlScriptGeneratorOptions, so this is a request against the library rather than against any particular editor integration.

Verified against main: the current Aligned output above matches the expected values in Test/SqlDom/ScriptGenerator/ClauseBodyAlignmentTests.cs (TestAlignedGroupByAndHaving).