Fixed inference for assign using PropertyAssigner

[Andarist]

fixes #3151

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[Andarist]

This has some drawbacks when it comes to inferring standalone assigns, like here:

const a1 = assign(/*...*/)

When Assigner is used then it works just fine:

const a1 = assign((ctx: { counter: number }) => ({})) // TContext inferred to `{ counter: number }`

but when it comes to PropertyAssigner then it doesn't infer TContext and it leaves it as unknown (the internal partial assigned function stays types correctly, it's about what the typeof a1 is):

const a1 = assign({
  counter: (context: { counter: number }) => context.counter + 1,
})

It also doesn't work, in this context, with static values:

const a1 = assign({
  counter: 100,
})

I think that LowInfer combined with reverse mapped types is broken in TS, a minimal repro case of this can be found here: TS playground