Add a mypy stub

[Ivan-Kouznetsov]

A mypy compatible stub file as per https://mypy.readthedocs.io/en/stable/stubs.html, using types annotations that mirror the comments in w2n.py.

This is needed to use this library in a project that uses mypy, otherwise the following error will be returned by mypy:
error: Skipping analyzing 'word2number': found module but no type hints or library stubs

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@33aac8a). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #62   +/-   ##
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  Coverage          ?   96.80%           
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  Files             ?        2           
  Lines             ?      125           
  Branches          ?        0           
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  Hits              ?      121           
  Misses            ?        4           
  Partials          ?        0           

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