Proper Output for BASE Function

Fix #4638. Microsoft does not document BASE as being introduced in Excel 2010 or later, but apparently it was.

src/PhpSpreadsheet/Writer/Xlsx/FunctionPrefix.php

@@ -66,6 +66,8 @@ class FunctionPrefix
         . '|var[.]s'
         . '|weibull[.]dist'
         . '|z[.]test'
+        // probably added with Excel 2010 but not properly documented
+        . '|base'
         // functions added with Excel 2013
         . '|acot'
         . '|acoth'

tests/PhpSpreadsheetTests/Writer/Xlsx/FunctionPrefixTest.php

@@ -38,6 +38,7 @@ class FunctionPrefixTest extends TestCase
             'DAYS/NETWORKDAYS 5' => ['NETWORKDAYS(DATE(2023,1,1),TODAY(), C:C)', 'NETWORKDAYS(DATE(2023,1,1),TODAY(), C:C)'],
             'COUNTIFS reclassified as Legacy' => ['COUNTIFS()', 'COUNTIFS()'],
             'SUMIFS reclassified as Legacy' => ['SUMIFS()', 'SUMIFS()'],
+            'BASE improperly classified by MS' => ['_xlfn.BASE()', 'BASE()'],
         ];
     }
 }