fix: Add end start and contains support for matchers. Fix import order. Add not_empty_string and extra main matchers

[MartinGotelli]

Add support for matchers in the is_string matcher:

def test_is_string_with_created_datetime_matcher():
    now = datetime.now()
    date_format = "%Y-%m-%d %H:%M:%S"
    now_string = now.strftime(date_format)
    assert now_string == is_datetime_string(date_format)
    start_text = f"{now_string}: Martín created this awesome test"
    assert start_text == is_string(starts_with=is_datetime_string(date_format))
    end_text = f"Someone decided to add some extra text to the end on: {now_string}"
    assert end_text == is_string(ends_with=is_datetime_string(date_format))
    contains_text = f"I once ate a sandwich, was on {now_string}, or maybe two"
    assert contains_text == is_string(contains=is_datetime_string(date_format))
    assert start_text == is_string(contains=is_datetime_string(date_format))
    assert end_text == is_string(contains=is_datetime_string(date_format))

Add not_empty_string, is_int, is_float, is_iso8601_date, is_iso8601_datetime, is_iso8601_time matchers