TYPO3.Fluid

One major source of errors when working with Fluid is the missing error handling when referring to a ViewHelper namespace that hasn't been declared.
Especially when one extracts parts of the template to a partial the namespace declaration is easily forgotten.

The issue with this is that namespace prefixes are allowed in xml so Fluid can't determine whether <unknown-namespace:foo /> is a mistyped ViewHelper or a (possibly valid) XML/XHTML tag that has been registered otherwise.
Given the importance of this we should solve this even though it would be a breaking change. We came up with following solution:

 1{namespace x=Some\Package\ViewHelpers}
 2{namespace xs}
 3
 4<f:foo />  <!-- as before: this would be a valid fluid tag as "f" is always registered (even though it would throw an exception because there is no VH "FooViewHelper" in the Fluid package -->
 5
 6<x:some.viewHelper />    <!-- as before: would resolve to \Some\Package\ViewHelpers\Some\ViewHelperViewHelper -->
 7
 8<xs:element />     <!-- as before but explicit: would be ignored, because the namespace is registered but does not point to a PHP namespace -->
 9
10<unknown:foo />     <!-- new: would throw an exception in Dev context, ignore the tag in Production context because the namespace "unknown" is not defined -->
11

When working with complex XML based files with a lot of namespaces (like XML Schema or LaTeX) it would be tedious to register all the namespaces so we should also support some kind of placeholder syntax like:

1{namespace *} <!-- ignores all unknown namespaces (old behavior) -->
2{namespace xs*} <!-- ignores all unknown namespaces that start with "xs" -->

Note: a different approach to this would be to configure namespaces not in the Fluid template but in the configuration. See #35766 and the related issues for reasons why we shouldn't do this