Refactor linting utilities to own submodule

[mshafer-NI]

As part of preparing for #80, splitting the refactor changes out.

Overview:

- _acknowledge_existing_errors._InMultilineStringChecker 
+ _utils.string_helpers.InMultilineStringChecker

- _acknowledge_existing_errors.DEFAULT_ENCODING
+ _utils.DEFAULT_ENCODING

- _acknowledge_existing_errors._get_lint_erros_to_process
+ _utils.lint.get_errors_to_process (also add excluded_errors as param)

- _acknowledge_existing_errors._lint_errors_parser.parse
+ _utils.lint.parse

- _acknowledge_existing_errors._remove_auto_suppressions_from_file 
+ _acknowledge_existing_errors.remove_auto_suppressions_from_file


- tests.test_cli.test_acknowledge_existing_errors.test_can_accurately_detect_if_in_multiline_string
+ tests.test_cli.test_utils.test_can_accurately_detect_if_in_multiline_string