Provides faster json decoding through simdjson bindings for PHP

Provides faster json decoding through simdjson bindings for PHP (Single Instruction, Multiple Data)

* Make the `SIMDJSON_ERR_*` constants case-sensitive in all PHP versions.
(The code it was based on was missing the flag needed to mark constants as case sensitive before PHP 8)
* Fix a bug that prevented using JSON pointer in `simdjson_key_count`, `simdjson_key_exists`, and `simdjson_key_value` with a leading slash https://www.rfc-editor.org/rfc/rfc6901.html.

This bug was introduced when working around test failures following a change in json pointer validation in the underlying C simdjson library.
* "" in a JSON pointer continues to refer to the entire document.
* "/" in a JSON pointer now properly refers to the key that is the empty string.
* Continue to allow the non-standard omission of the leading "/" for compatibility with earlier PECL releases. This may be deprecated in a subsequent release.