Fix qwen3 template parsing, implement jinja slice step syntax

[matteoserva]

This implements the steps needed to parse messages[::-1] in Qwen3 template.
Fixes #13178

Sending same patch to upstream

[CISC]

Sorry, but this does not implement the step parameter, look more closely at the code (and more specifically at SubscriptExpr).

Also, the template will still fail due to the startswith method.

[matteoserva]

ok. basically useless. closing