Symbol properties should not be exempt from excess property checks in presence of a string index signature

In #44793, we found that a symbol index signature doesn't turn off excess property checks on symbols; however, a string index signature turns these excess property checks entirely.

interface A {
    a?: string
    b?: number
    [key: string]: unknown
}

const symbolKey = Symbol('key')

const a: A = {
    [symbolKey]: '123', // works!
}

It's likely that some users relied on this behavior; however, those users will be able to use symbol index signatures in TypeScript 4.4.