Optimize JSON pretty print indentation performance (GH-21474)

Grow the string buffer in advance, avoiding repeated reallocations for each indentation level.

Author: LamentXU123

UPGRADING

@@ -252,6 +252,8 @@ PHP 8.6 UPGRADE NOTES
 
 - JSON:
   . Improve performance of encoding arrays and objects.
+  . Improved performance of indentation generation in json_encode()
+    when using PHP_JSON_PRETTY_PRINT.
 
 - Standard:
   . Improved performance of array_fill_keys().

ext/json/json_encoder.c

@@ -54,6 +54,7 @@ static inline void php_json_pretty_print_char(smart_str *buf, int options, char
 static inline void php_json_pretty_print_indent(smart_str *buf, int options, const php_json_encoder *encoder) /* {{{ */
 {
 	if (options & PHP_JSON_PRETTY_PRINT) {
+		smart_str_alloc(buf, encoder->depth * 4, 0);
 		for (int i = 0; i < encoder->depth; ++i) {
 			smart_str_appendl(buf, "    ", 4);
 		}