Add namespace support to templates

[HLeithner]

Replacement PR for #30816 the original author deleted the PR.

Summary of Changes

Add the namespace support to the template manifest file.

Testing Instructions

• Test existing templates
• Test existing child template
• Test copying old templates without namespace
• Test new dummy template
  • no copy function

Test in Frontend and Backend

Here are some example template to test all 4 variants:
namespacedummy.zip
namespacedummynoparent.zip
nonamespacedummy.zip
nonamespacedummynoparent.zip

Actual result BEFORE applying this Pull Request

No Namespace support for templates

Expected result AFTER applying this Pull Request

Namespace autoloading for templates

Link to documentations

Please select:

• Documentation link for docs.joomla.org: Modifying Template, don't know if we have a j4 version of this

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org: Add template namespace support to Joomla 4.3 Manual#47

• No documentation changes for manual.joomla.org needed

[ReLater]

I have tested this item ✅ successfully on e047e46

Just BTW: Found this issue during tests. After investigations: Not related with this PR here. #39329

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[dgrammatiko]

I have tested this item ✅ successfully on e047e46

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/39011.}

[dgrammatiko]

RTC

[richard67]

RTC

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[bembelimen]

Thx