Number literal object keys variance annotation parsing

[goodmind]

Related #7593

It's impossible to have negative number key, so it can only mean contravariant annotation

const bar = {-1: 'foo'}

VM153:1 Uncaught SyntaxError: Unexpected token -

type X = {-1: T}
type X = {+1: T}
type X = {1: T}

[goodmind]

@mroch I think this is related to number keys and #380

[goodmind]

@mroch can you look into this?