Support lower-case meridiem

[fsoikin]

Description of the change
Adds support for lower-case meridiem (i.e. "am" or "pm" instead of "AM" and "PM") via a new FormatterCommand constructor MeridiemLowerCase and its string representation "A".

This is something our designers really want to have, and so far we have been doing a combination of ignoring the designers' requests and/or separately lowercasing the resulting string. The latter is not always possible, e.g. in case the string contains month or week day name, which can be worked around by formatting those separately and concatenating, but this kind of defeats the point of using a formatter library in the first place.

The print-parse-print loop doesn't work - it will lose the lower-case variant upon parsing, - but then it never did work in this respect, so no harm.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• N/A Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)