Mathics-scanner

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Tag unicode operators which have no definition and add the ability to dump them. This is useful for mathics-pygments.

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* Finish adding operator names.
* Generate ``IndentingNewline`` (``\n``) properly in GNU Readline inputrc tables.
* Adjust expectation on test since there can be duplicate function operators (for ``Apply`` and ``Function``).

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* Add operator-name, and ASCII fields. See named-characters.yml for a description of these
* Add some whitespace characters like IndentingNewLine and RawReadLine
* Improve testing
* Fix some small tagging based on testing
* Add unicode-to-operator generation

Note: not all operators have been tagged, so expect another release soon when that's done.

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* The scanner split off from Mathics3.
* Tables added for converting between WL names and Unicode, ASCII and character properties. See ``implementation.rst`` for details.
* Code and docstring gone over.

See git in Mathics before Jan 18, 2021 for old history of these files.