Add '.' (period, dot, ..) to FormatLiterals so that ss.fff can work. (#23861)

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.

lib/pure/times.nim

@@ -1631,7 +1631,7 @@ const
         "Sunday"],
   )
 
-  FormatLiterals = {' ', '-', '/', ':', '(', ')', '[', ']', ','}
+  FormatLiterals = {' ', '-', '/', ':', '(', ')', '[', ']', ',', '.'}
 
 proc `$`*(f: TimeFormat): string =
   ## Returns the format string that was used to construct `f`.