json_encode does not return false if JSON_THROW_ON_ERROR is in flags

[orklah]

This fixes #2996

A few caveats:

• JSON_THROW_ON_ERROR appeared on PHP 7.3. If anyone push the literal 4194304 in PHP 7.2, the return type will be funky
• I didn't use int-mask because there are 29 JSON_* constants and I thought Psalm would not like that. The consequence is that if someone combine flags, The return type will go back to |false

[AndrolGenhald]

* JSON_THROW_ON_ERROR  appeared on PHP 7.3. If anyone push the literal 4194304 in PHP 7.2, the return type will be funky

Would a conditional return type with something like PHP_MAJOR_VERSION > 7 || PHP_MAJOR_VERSION === 7 && PHP_MINOR_VERSION >= 3 fix that? I haven't used them much so I'm not sure how much logic is supported.

Edit: NVM, looks like only an is operator is supported.

[orklah]

Yeah, this is possible, but given you can't use &&, you have to create branches for everything. It'll make a big mess for small gains. Especially if we want to handle more flags in the future

[muglug]

Thanks!