implement gettype() inference in TypeSpecifier

[staabm]

closes phpstan/phpstan#6901

[ondrejmirtes]

Please test some more scenarios.

1. Three equal signs: if (gettype($x) === 'int')
2. Negative context:

// $x is int|string|bool
if (gettype($x) === 'integer') {
    // assert int
    return;
}

// assert string|bool

3. BooleanOr:

if (gettype($x) === 'string' || gettype($x) === 'integer') {
    // assert int|string
}

[staabm]

great feedback thx. added tests and simplified the implementation based on it :)

[ondrejmirtes]

Thank you!