Honestly, to me the entire design of a (highly!) restricted set of
`FormatLiterals` characters seems antithetical to the very idea of a
format string template. Fixing that is a much larger change, though.
So, this PR just adds `'.'` so that the standard (both input & output!)
notation for decimal numbers in Nim can be used for the seconds part of
a time format in `lib/pure/times.format(.., f)`. It should only make
legal what was illegal and should be harmless since `'.'` is not used in
any special way otherwise.