Added ClassConstants filter

[siad007]

Assembles the constants declared in a PHP class in

key1=value1
key2=value2

format.
Example usage:

class Foo
{
    public const CONST1 = 'CONST 1';
    protected const CONST2 = 'CONST 2';
    private const CONST3 = 'CONST 3';
}

<property file="constants.php">
    <filterchain>
      <classconstants />
    </filterchain>
</property>
<echo msg"${CONST1}"/>
<echo msg"${CONST2}"/>
<echo msg"${CONST3}"/>

[codecov]

Codecov Report

Merging #1322 into master will increase coverage by 0.04%.
The diff coverage is 96.29%.

@@             Coverage Diff              @@
##             master    #1322      +/-   ##
============================================
+ Coverage     51.79%   51.84%   +0.04%     
- Complexity     9245     9255      +10     
============================================
  Files           474      475       +1     
  Lines         22593    22618      +25     
============================================
+ Hits          11703    11727      +24     
- Misses        10890    10891       +1     

Impacted Files	Coverage Δ	Complexity Δ
classes/phing/filters/ClassConstants.php	93.33% <93.33%> (ø)	5.00 <5.00> (?)
classes/phing/tasks/system/PropertyTask.php	81.00% <100.00%> (+0.88%)	80.00 <0.00> (+4.00)
classes/phing/types/FilterChain.php	52.85% <100.00%> (+1.38%)	33.00 <1.00> (+1.00)

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