[FEATURE] Proper support of local variables in ViewHelpers

[s2b]

In its current implementation, the f:alias ViewHelper has side effects: It adds variables to the global variable container, which are not restored properly after the ViewHelper call. If a variable name is used as alias which already exists, that variable is unset after the ViewHelper call.

To resolve this, a new ScopedVariableProvider is introduced to handle local variables
inside ViewHelper calls properly. After the local scope, global variables are restored to their original value in case they existed. At the same time, other modifications to the variable provider inside the f:alias call, such as a call to f:variable, should still be persistent after the ViewHelper call. This ensures that the change is non-breaking for the vast majority of usages. The only situation where this could be breaking is when you relied on the arguably broken behavior that f:alias unsets the variables afterwards.

This is also the reason why we can't simple store and reset the whole variable container. This would be a breaking change and would also be a diversion from other ViewHelpers, such as f:if, where f:variable also leaks out. To change this behavior for select ViewHelpers would be inconsistent and hard to understand for users of Fluid.

before:

<f:variable name="myVariable" value="initialValue" />
<f:alias map="{myVariable: \'myValue\'}">...</f:alias>
{myVariable} <!-- outputs empty string -->

after:

<f:variable name="myVariable" value="initialValue" />
<f:alias map="{myVariable: \'myValue\'}">...</f:alias>
{myVariable} <!-- outputs "initialValue" -->

This behavior also applies to other ViewHelpers that provide variables to their child nodes, such as f:for, f:render, f:groupedFor, f:cycle. These will be modified accordingly in separate PRs.

Resolves #666

[mbrodala]

Only a few words: bam! 💥

[mbrodala]

Thanks a lot for this missing piece!