[Str] add `after` and `before` helpers about psl HOT 2 CLOSED

I'm not sure what after and before are supposed to do, to be honest. Is the idea that

• Str\after('hello world', 'w') === 'orld'
• Str\after('hello world', 'w', true) === 'world'
• Str\after('hello world', 'w', true, 2) === 'rld'
• Str\after('hello world', 'w', false, 2) === 'ld'
• Str\before('hello world', 'o') === 'hell'
• Str\before('hello world', 'o', true) === 'hello'
• Str\before('hello world', 'o', true, 2) === 'hello w'
• Str\before('hello world', 'o', false, 2) === 'hello '

I understand based on hack that _last would indicate using the last occurrence of $needle in $haystack and _ci indicates case insensitive, but wasn't sure if I understood how you wanted after/before to work, and whether the offset was used as a length of to offset from $needle or what.

from psl.

your understanding is correct, but $offset is not used as a length, but as an $offset since these functions would use search, search_last, search_ci, and search_last_ci internally, the $offset argument should be passed to the search function.

see: after, afterLast, before, and beforeLast at https://github.com/symfony/string/blob/master/AbstractString.php#L130

from psl.