Bug in costructor typing

```
new MetaMaskOnboarding({
forwarderOrigin: 'http://localhost:3000',
forwarderMode : 'OPEN_TAB'
})
```

throws : `Type '"OPEN_TAB"' is not assignable to type '"INJECT"'.ts(2322)`
Which is consequnce of bad typing of constructor.

Guess you wanted to have something like

```
export default class Onboarding {
static FORWARDER_MODE = {
INJECT: 'INJECT' as const,
OPEN_TAB: 'OPEN_TAB' as const,
};

constructor({
forwarderOrigin = 'https://fwd.metamask.io',
forwarderMode = Onboarding.FORWARDER_MODE.INJECT,
} : {
forwarderMode? : keyof typeof Onboarding.FORWARDER_MODE,
forwarderOrigin? : string
}) {
}
}```

which provides defaults but same time gives proper types for constructor. Should i open PR?

Start at the TypeScript Onboarding constructor and its FORWARDER_MODE definition, as shown in the issue. Reproduce the reported OPEN_TAB type error, then verify that both forwarder modes are accepted while the defaults remain available and the constructor type-checks successfully.

Written by the indexing model from the issue text.