Add missing Float#weeks method similar to Int#weeks

[vlazar]

Both Int and Float have days, hours,minutes, seconds, milliseconds, microseconds, nanoseconds.

On top of that Int also has weeks method, but Float does not.

[j8r]

Not directly related to this PR - it starts to have too many singular/plural aliases related to Timpe::Span 😟
I'll vote to keep the plural form, which is consistent with Int#times, and the method in Float which are all plural (thus for 1.0.days)

[wooster0]

Isn't Float#months and Float#years missing as well?

[vlazar]

@r00ster91 You are right, but these return Time::MonthSpan, not Time::Span like all other methods (weeks .. nanoseconds) and Time::MonthSpan stores values as Int64, so unless that's changed the months and years for Float wouldn't make much sense.

[vlazar]

Maybe there is a reason behind Time::MonthSpan not using floats. And if not, changing Time::MonthSpan implementation feels like a bigger change than just adding missing method(s). I guess it's better to discuss it first and address in separate PR. But I'm happy to address it here, if there is a need for it of course.

[straight-shoota]

MontSpan can't express a fraction of a month, that's why float months and years don't make much sense.
This requires an appropriate datatype, see #6522.