Add new Rector rules and improve compatibility

[spawnia]

Summary

• Add IfThrowToCoalesceThrowRector to transform null-check-then-throw patterns to use the null coalesce operator with throw expression
• Add ElvisToCoalesceRector to convert elvis operator (?:) to null coalesce (??) when the expression can only be falsy via null
• Select FirstClassCallableRector or ArrayToFirstClassCallableRector based on Rector version
• Fix Rector 1.x compatibility for IfThrowToCoalesceThrowRector

Example

Before:

$value = getValue();
if ($value === null) {
    throw new Exception('Value required');
}

$foo = $bar ?: 'default';

After:

$value = getValue() ?? throw new Exception('Value required');

$foo = $bar ?? 'default';

Test plan

• PHPStan passes
• Tests pass

🤖 Generated with Claude Code

[spawnia]

Formatting will be handled by mll-lab/php-cs-fixer-config#14.

[spawnia]

Added ElvisToCoalesceRector which converts ?: to ?? when the expression type can only be falsy via null (e.g., ?\stdClass).

Also improved IfThrowToCoalesceThrowRector to prefer ?? over ?: when type analysis shows this is safe. The shared type-checking logic is extracted into NullFalsyTypeTrait.

@simbig could you re-review?