Improve Option::select() return type

[axlon]

This change adds a minor improvement to the return type of Option::select() for static analysis.

/** @var \PhpOption\Option<1|2|3> $option */
-PHPStan\dumpType($option->select(2)); // PhpOption\Option<1|2|3>
+PHPStan\dumpType($option->select(2)); // PhpOption\Option<2>

[GrahamCampbell]

Thanks for the PR, but I wasn't able to reproduce the improvement: on current PHPStan (1.12 and 2.x alike) and Psalm, select(2) on Option<1|2|3> still resolves to Option<1|2|3>, because the inferred template type is generalized back to its bound, so this change has no observable effect. If analyzers ever did start honoring the bound, the narrowing would interact badly with Option's invariant T and produce new errors wherever a select() result flows back into an Option<T> position. Closing, since as it stands this adds a latent behaviour change without a demonstrable benefit.