Add shortcut for `CASE` statements

[elpete]

Need to think about the syntax since queries can go on either side, the condition or the expression. Probably accept literals or closures or builder instances or raw expressions.

Also need to validate that this is a good idea.

[elpete]

Syntax Idea:

qb.case( ( c ) => {
	c.whenExists( ( q ) => {
		q.selectRaw( 1 )
			.from( "table_one" )
			.whereColumn( "table_one.baseId, "base_table.id" )
			.whereExists( ( q2 ) => {
				q2.selectRaw( 1 )
					.from( "table_two" )
					.whereColumn( "table_two.oneId", "table_one.id" )
					.where( "table_two.id", 1 );
			} )
	}, 1 );
	c.whenRaw( "id IN (1,2,5)", 4 );
	c.else( 0 );
} );

[elpete]

For now, raw SQL remains the preferred way to write CASE expressions. Keeping these design notes for future reconsideration:

A fluent, binding-aware API could mirror qb's existing predicate vocabulary:

builder
    .caseWhen( "status", "active" )
        .then( "Active" )
    .when( "status", "pending" )
        .then( "Pending" )
    .otherwise( "Inactive" )
    .as( "statusLabel" )

Potential initial scope:

• when( column, value ) and when( column, operator, value )
• whenColumn()
• whenNull()
• whenIn()
• grouped conditions through a callback using normal qb where methods
• otherwise()
• return a binding-aware expression usable in selects, ordering, grouping, and other places that accept expressions

More advanced forms such as whenExists, nested cases, simple CASE value WHEN ... syntax, and aggregate helpers could be deferred.

The portability benefit is primarily qb-managed identifier quoting, bindings, predicate compilation, subqueries, and binding order. The core CASE WHEN ... THEN ... ELSE ... END syntax is already portable. Conditions supplied through whenRaw() would remain database-specific.