Add Ternary operator mutator

[sidz]

This PR:

• Adds new Ternary mutator
• Covered by tests
• Schema needs an update
• Doc PR: Update mutators.md with Ternary mutator site#186

Mutates:

- isset($b) ? 'B' : 'C';
+ isset($b) ? 'C' : 'B';

$foo = 'foo';
- $foo ?: 'bar';
+ $foo ? 'bar' : $foo;

[sidz]

PrettyPrinter from PhpParser generates nested ternary operators instead of wrap them up in braces. Fixed in nikic/PHP-Parser@eff72ee and hope will be in the next release so we will be able to remove this condition.

Nested ternary is deprecated since 7.4 and dropped in 8.0

[maks-rafalko]

👌