[Issue 18856] [std.experimental.logger] please add LogLevel.debug

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18856

--- Comment #1 from anonymous4 <dfj1esp02 at sneakemail.com> ---
`debug` is a keyword and can't be used for an identifier, I suppose
LogLevel.trace serves the purpose of debug log level.

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