FEAT Add FirstLetterConverter

[fdubut]

Description

This PR adds a converter that replaces each word of the prompt with its first letter (or digit).

Design choices:

• Anything that's not a letter or digit is stripped from the tokens (split by space in WordLevelConverter).
• Hyphenated compounds (e.g. "mother-in-law") and contractions (e.g. "don't") are considered single words.
• First letters are joined without separator so that users can apply StringJoinConverter with the separator of their choice.

Tests and Documentation

Added unit tests, including multi-line prompts, French punctuation and Japanese characters.

I excluded the test script from flake8 because it contains UTF-8 characters that are not supporter by flake8-copyright.

I did not add this converter to the doc notebook that has other converter examples because it's a simple one.

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[hannahwestra25]

Thanks @fdubut ! I think when Blake and I tested this we used a single space in between characters. JW if you tested this with no spaces and saw that it worked ? We could optionally add a whitespace option for the converter in between characters