(typing) Allow readonly array of wallets for WalletProvider

[dearlordylord]

In mine and some other codebases, immutability goes by default.

The adapter doesn't like it, so the type like const wallets: readonly [PhantomWalletAdapter, SolflareWalletAdapter] won't work:

const useWallets = () => {
  const phantomWallet = new PhantomWalletAdapter();
  const solflareWallet = new SolflareWalletAdapter();
  return useMemo(() => [phantomWallet, solflareWallet] as const, [phantomWallet, solflareWallet]);
}

...

const wallets = useWallets();
<WalletProvider wallets={wallets}/>

I have to either cast or re-construct <WalletProvider wallets={[...wallets]}/>, which is undesirable.

This PR allows readonly arrays as wallets prop, as well as keeps mutable arrays backwards-compatible.