detector: add startswith matching for StringDetector base class

[leondz]

Adds a new match type for the StringDetector that fires when the output starts with any of the candidate match strings.

e.g. with "test" in the match strings:

    test_cases = [
        ("this is a test", False),
        ("tests always succeed", True),
    ]

Case sensitivity test also added. Thanks @Eric-Hacker for the implicit suggestion & test case in #1054 .

Verification

• python -m pytest tests/detectors/test_detectors_base.py