add phpredis stubs

[kkmuffme]

Fixes #7581

Source: https://github.com/phpredis/phpredis/blob/e93be87023fffbf9213e8f47066f6dd646d8ba5a/redis.stub.php
=> these are not fully correct though (return types are partly wrong, params are partly wrong).

However there is a pending PR with correct stubs by the phpredis maintainer with correct stubs:
phpredis/phpredis#2015
So I used the stubs from that PR as they are correct: https://github.com/phpredis/phpredis/blob/e93be87023fffbf9213e8f47066f6dd646d8ba5a/redis.stub.php

[AndrolGenhald]

For 4.x you'll have to add an extension_loaded check around here so that the stub is actually used.

[kkmuffme]

Thanks, added now.

[orklah]

Thanks!