Added simdjson stubs. #1918

Job	Run time
Run cs-fixer and stubs structure tests	2m 31s
Run tests against php 5.6	9m 46s
Run tests against php 7.0	9m 5s
Run tests against php 7.1	9m 54s
Run tests against php 7.2	10m 2s
Run tests against php 7.3	10m 59s
Run tests against php 7.4	11m 25s
Run tests against php 8.0	12m 9s
Run tests against php 8.1	12m 26s
Run tests against php 8.2	12m 29s
Run tests against php 8.3	12m 57s
	1h 53m 43s

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