Added a method to get user timezone as DateTimezone instance

[julienV]

Summary of Changes

I added function to the JUser to get the user timezone, which is very useful for any developer working with date and time in Joomla.
It's a simple function, but it already simplifies some cumbersome library calls.

Testing Instructions

Fill and save any form field using calendar field with 'user_utc', like article publish date for example

Expected result

The date and time saved should be the set date and time...

[phproberto]

I have tested this item ✅ successfully on 99c9e47

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[joomdonation]

I have tested this item ✅ successfully on 99c9e47

Code review + write code JFactory::getUser()->getTimeZone(); to check the result. It is working as expected.

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/15007.}

[ghost]

I have tested this item ✅ successfully on 99c9e47

Changed a given Date-Time-Format to user_utc give after saving Article same Date-Time-Format before saving.

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/15007.}

[ghost]

RTC after 3 successful testes.

[ghost]

@joomdonation was quicker than me.

[joomdonation]

@franz-wohlkoenig No problem if there are more than two tests per PR :D

[julienV]

thanks ;)

[ghost]

@joomdonation if you test #15021 we have one more for RTC ;-)

[Bakual]

Can you add a @since __DEPLOY_VERSION__ to the doc block?

[julienV]

done