fix: import for State, for using State Schema, is missing in streamsync highlevel import

[FabienArcellier]

fix #369

You have to write ss.core.State instead of ss.State

import streamsync as ss

class MyappSchema(ss.core.State):
    title: str

class AppSchema(ss.StreamsyncState):
    my_app: MyappSchema
    counter: int

initial_state = ss.init_state({
    "counter": 0,
    "my_app": {
        "title": "Nested value"
    }
}, schema=AppSchema)