Add utility method RipGrepOut.files_list

[vgadoury]

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Add RipGrepOut.files_list as and utility result converter for when using the files...() (or count...()) options.

Could be considered as suger, basically equivalent to self.as_string.split("\n")[:-1], but with error handling and support for --null terminator.

Example:

>>> from ripgrepy import Ripgrepy
>>> res = Ripgrepy("Ripgrepy", "./ripgrepy/").files_with_matches().run()
>>> res.files_list
['./ripgrepy/__init__.py']

Note: this does not list the files that match for regular search cases (with lines results and all), and throws an explicit TypeError if used with unsupported command arguments.

[RecRanger]

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