[TASK] Use native type declarations in ParserState

[oliverklee]

Part of #811

[coveralls]

coverage: 55.567%. remained the same
when pulling e71c3d7 on task/types/parser-state
into db8cad1 on main.

[JakeQZ]

$offset for strpos can be negative. Otherwise changes look fine.

[JakeQZ]

$offset can be negative to indicate an offset from the end of the string. (Note that the version of substr provided in this class does not support a negative offset, whereas this method does because it calls on to the built-in PHP functions.)

[JakeQZ]

We can lose the @param in the DocBlock for $offset now.