[CodeQuality] Skip alternative syntax (if/endif) in ShortenElseIfRector

[TomasVotruba]

Fixes rectorphp/rector#9803

ShortenElseIfRector produced invalid PHP when a file used the alternative control syntax (if: / else: / endif). It merged else { if } into elseif regardless of syntax style: the freshly created ElseIf_ node prints braces while the surrounding unchanged nodes keep their colons, yielding mixed brace/colon output that fails to parse (and drops the trailing endif).

Before (broken output)

if ($n == $c) {
    $a = 1;
} elseif ($x || $n <= $e) {
    $a = 2;
} elseif ($d && ! $x) :
    $a = 3;

PHP Parse error: syntax error, unexpected token :

After

Files using alternative syntax are skipped, both when the outer if uses it and when only the nested if does. Normal brace syntax is unchanged:

 if ($cond1) {
     return $action1;
-} else {
-    if ($cond2) {
-        return $action2;
-    }
+} elseif ($cond2) {
+    return $action2;
 }

Detection reuses the same getOldTokens() token-scan approach as the sibling CompleteMissingIfElseBracketRector: scan tokens after the condition and skip if : appears before {.

Two skip_* fixtures added (full alternative syntax + brace-outer/alt-inner).