[9.x] Add methods to cast Stringables

[jasonmccreary]

This adds methods to conveniently cast stringables to common data types similar to those found in Laravel's HTTP Request.

Before

intval(str('shift-worker-01')->afterLast('-')->toString());
floatval(str('Result: 1.23')->after(':')->trim()->toString());
str('YeS')->lower()->toString() === 'yes';
Carbon::parse(str('DOB: 12-31-2001')->after(':')->trim()->toString());

After

str('shift-worker-01')->afterLast('-')->toInteger();
str('Result: 1.23')->after(':')->trim()->toFloat();
str('YeS')->lower()->toBoolean();
str('DOB: 12-31-2001')->after(':')->trim()->toDate();