JSON: Add opt-in comments and trailing comma flags

[otar]

Configuration files often use a narrow JSON extension with JS-style comments, trailing commas, or both. PHP currently requires a separate parser or a string-aware preprocessing pass. This PR adds native parser support. Strict JSON remains the default.

Public API

Constant	Value	Accepted by	Effect
JSON_ALLOW_COMMENTS	1 << 23	json_decode(), json_validate()	Allows // and non-nesting /* ... */ comments at JSON whitespace boundaries.
JSON_ALLOW_TRAILING_COMMAS	1 << 24	json_decode(), json_validate()	Allows one final comma in a nonempty array or object.

The flags are independent. Neither flag applies to json_encode() or is enabled by default.

Semantics

• Line comments end at LF, CRLF, lone CR, or EOF. Block comments do not nest and end at the first */.
• Comments are allowed before and after the root value and between tokens. They are not allowed inside a token. Comment-like text in a JSON string is unchanged.
• U+2028/U+2029 and valid controls, including embedded NUL, are comment content. Malformed UTF-8 follows the existing decode flags. Validation still accepts JSON_INVALID_UTF8_IGNORE, but not JSON_INVALID_UTF8_SUBSTITUTE.
• Unterminated block comments produce JSON_ERROR_SYNTAX at the opening slash.
• A trailing comma is allowed only after the last element or member of a nonempty container. Empty or missing-element forms such as [,], {,}, and [1,,] remain invalid.
• The implementation retains existing behavior for depth, duplicate keys, bigints, associative objects, exceptions, error codes, and original-input line and column numbers.

[otar]

Sorry @bukka and @kocsismate, I accidentally tagged you on this PR. Please ignore this draft...