Generic/DisallowTabIndent: do not auto-fix heredoc/nowdoc closer indent

Follow up on squizlabs/PHP_CodeSniffer 3640, which added the `T_END_HEREDOC` and `T_END_NOWDOC` tokens to the list of tokens to examine and was included in the 3.7.2 release.

When a flexible heredoc/nowdoc is used, the indentation type of the heredoc/nowdoc content and the indentation type of the heredoc/nowdoc closer must be consistent, so auto-fixing the indentation of the closer, without also changing the indentation of the contents (from tabs to spaces) would cause a parse error.
See: https://3v4l.org/7OF3M

I do believe indentation tabs should still be flagged, but the auto-fixer should be disabled.

To allow people to disregard tab indentation for heredoc/nowdoc closers completely, the error now also has a more specific error code - ` TabsUsedHeredocCloser` -, which allows for excluding it in a ruleset.

While the change in the error code _could_ be considered a breaking change, the fact that the potential for a parse error hasn't been reported as a bug so far, leads me to believe this is not a frequently encountered situation, so the impact of the higher specificity for the error code should be minimal.

src/Standards/Generic/Sniffs/WhiteSpace/DisallowTabIndentSniff.php

@@ -169,6 +169,14 @@ public function process(File $phpcsFile, $stackPtr)
                 continue;
             }
 
+            // Report, but don't auto-fix tab identation for a PHP 7.3+ flexible heredoc/nowdoc closer.
+            // Auto-fixing this would cause parse errors as the indentation of the heredoc/nowdoc contents
+            // needs to use the same type of indentation. Also see: https://3v4l.org/7OF3M .
+            if ($tokens[$i]['code'] === T_END_HEREDOC || $tokens[$i]['code'] === T_END_NOWDOC) {
+                $phpcsFile->addError($error, $i, $errorCode . 'HeredocCloser');
+                continue;
+            }
+
             $fix = $phpcsFile->addFixableError($error, $i, $errorCode);
             if ($fix === true) {
                 if (isset($tokens[$i]['orig_content']) === true) {