Optimize JSON pretty print indentation performance

[LamentXU123]

This PR optimizes the php_json_pretty_print_indent function in the JSON extension to improve performance when encoding structures with JSON_PRETTY_PRINT.

When I was reading this, I found that now, the indentation logic used a for loop to append spaces in increments of 4 characters per depth level. For a JSON structure with a depth of N, this resulted in N consecutive calls to smart_str_appendl. This approach introduces unnecessary overhead due to repeated function calls.

So I think I would introduce this space-time optimization to add a static constant string of pre-allocated spaces. For almost all typical JSON depths, this reduces the number of smart_str_appendl calls from O(N) to exactly 1. This significantly reduces function call overhead and improves CPU performance during json_encode with JSON_PRETTY_PRINT, especially for deeply nested data.

[staabm]

This significantly reduces function call overhead and improves CPU performance during json_encode with JSON_PRETTY_PRINT, especially for deeply nested data.

Could you give some before/after numbers?

[LamentXU123]

This significantly reduces function call overhead and improves CPU performance during json_encode with JSON_PRETTY_PRINT, especially for deeply nested data.

Could you give some before/after numbers?

Sure do, but maybe tomorrow. I think this is a pretty obvious optimization so I don't do this initially

[iluuu1994]

A short benchmark would be appreciated. smart_str_appendl() is inlined, so whether this actually improves performance is hard to predict.