bpo-44722: Added a Multiline Formatter

[creative-resort]

RFC: string Multiline Formatter

I'd like to contribute a multiline formatter to the string library.
This is a first draft, that I've so far done basic testing for.

Please let me know your thoughts. I am sure it can be further improved.

The idea:

Format strings

• that are comprised of field values with newlines (Multiline)
• as Multiline strings in a quasi tabbed format.

This is in particular useful for logging of information, that occupies multiple lines.

For example:

Given the format string '{levelname}:{name}:{message}:{api}',
where the message and the api field is a string with multiple lines (contains newlines) ...

Result:

... Is formatted as:

2021-07-23 09:50:28,981 module_name WARNING     Quota exceeded         Google-Cloud-API
                                                Backing off for 5s     version: v1
                                                after trying 2x

https://bugs.python.org/issue44722

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