feat: Add optional regex engine

[KlfJoat]

• Expand assert_file_not_contains to allow an optional regex engine selection, to mirror assert_file_contains.

• Change README to reflect this change.

• Since there were no regex engine tests for assert_file_contains, I did not add any for my expanded assert_file_not_contains.

[KlfJoat]

Once this is approved, I will also expand the allowed regex engines in both assertions. I have found zgrep, zegrep, and zipgrep to be useful regex engines. I've modified bats-file locally to include them, and have found them useful over the past months.

Especially zipgrep, since a common modern method for creating application files is to ZIP up a collection of text files. This is used by EPUB, OpenDocument (ODT, ODS, etc), Office Open XML / OOXML (DOCX, XLSX, etc), and a few others where a simple grep MAY not find text in the file.

I do wonder if there is a way to create tests for these engines. But it might require modifying the test environment more than you find acceptable.