Add Qiskit Textbook circuit drawer style

[frankharkins]

Summary

We have been using a custom circuit drawer on the Qiskit Textbook for a while now. This PR makes it part of Qiskit itself to make it easier for users and contributors to reproduce our diagrams.

Details and comments

Only list of styles I could see was in the qiskit.visualization.qcstyle.DefaultStyle docstring, which I updated.

Color scheme designed by Grace Lindsell and Russell Huffman.

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[HuangJunye]

Sounds great! I didn't know the textbook use a different style. Can you provide a few links to examples to see the style? It would be good to put a screenshot in the PR just for the record as well.

[enavarro51]

There are specific 'mpl' snapshot tests for "iqx", "bw", and "iqx-dark", so it would be a good idea to add one for "textbook" as well. You can use the "iqx" one as a starter. If there are specific gates that your style displays differently from "default" and are not on the "iqx", you should probably add them to the display. If you haven't done one before, there are guidelines here, https://github.com/Qiskit/qiskit-terra/blob/main/CONTRIBUTING.md#snapshot-testing-for-visualizations.

[jakelishman]

One American localisation comment, but other than that this looks fine to me - I wouldn't want us to get in the habit of supplying a lot of themes, but the very public ones we use in our own documentation are probably sensible.

[coveralls]

Pull Request Test Coverage Report for Build 2460622816

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.005%) to 84.411%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/pulse/library/waveform.py	3	89.36%

Totals
Change from base Build 2454743017:	-0.005%
Covered Lines:	54666
Relevant Lines:	64762

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💛 - Coveralls

[jakelishman]

Looks good, thanks for the test!