[FEATURE] Add new ViewHelper f:variable via Fluid 2.2.0

A new Fluid ViewHelper, `f:variable`, has become available
in TYPO3 after raising the minimum required version of the
Fluid engine to 2.2.0.

Change-Id: I4f42030ef586e8d9a3d40c54c64137eb1969384d
Resolves: #79402
Releases: master
Reviewed-on: https://review.typo3.org/51398


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Tested-by: Thomas Hohn <thomas@hohn.dk>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>

typo3/sysext/core/Documentation/Changelog/master/Feature-79402-VariableViewHelperForFluid.rst

+.. include:: ../../Includes.txt
+
+=======================================================
+Feature: #79402 - New Fluid ViewHelper f:variable added
+=======================================================
+
+See :issue:`79402`
+
+
+Description
+===========
+
+A new ViewHelper ``f:variable`` has been added in Fluid 2.2.0 which is now minimum required dependency for TYPO3.
+The ViewHelper allows variables to be assigned in the template:
+
+.. code-block:: html
+
+    Assign a variable
+
+    <f:variable name="myvariable">My variable's content</f:variable>
+    <f:variable name="myvariable" value="My variable's content"/>
+    {f:variable(name: 'myvariable', value: 'My variable\'s content')}
+    {myoriginalvariable -> f:variable.set(name: 'mynewvariable')}
+
+Impact
+======
+
+The new ViewHelper is now available in any and all Fluid templates being rendered in TYPO3.
+
+.. index:: Fluid