Improve SSE reconnection logic

[Telroshan]

Extension changes

• Rebind children SSE listeners on reconnection as a new event source was created. Current implementation makes the reconnection process a bit useless as all existing elements won't work anymore anyway, only new content swapped in would benefit from the reconnected event source
• Fix retry delay increment by power of 2 (current implementation is actually a XOR, not a pow operation)
• Reset delay on successful connection (only increase delay for consecutive failed connections)

Also added tests for the reconnection process + retry timeout increase & reset

Testing mock changes:

• Made the actual "connection" happen 1ms after the connect call, to let the SSE extension bind its onopen and onerror callbacks in the meantime, as it would do in a real situation
• Added a readyState property to mirror the EventSource.readyState property, with the same 3 enum values, instead of a simple "wasClosed" bool
• Connection failure simulation

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