[Request] Move into `polkadot-sdk` monorepo

[kayabaNerve]

substrate-bip39, by name, is BIP-39 for substrate. While substrate used to have its own repo, which I'd then request substrate-bip39 to be included under if it was still around, substrate now lives under the polkadot-sdk monorepo.

I don't see why substrate-dedicated repos should exist outside the monorepo. It places less attention on them and requires multiple PRs to trigger updates. While this code is largely stagnant (possibly in some part due to lack of attention), which minimizes how often said multiple PRs occur, I have had to write updates for it and it's personally annoying to have to maintain forks of substrate-bip39 and polkadot-sdk. I wouldn't raise this request over solely the personal annoyance though, and believe it legitimately proper to be in monorepo.

Please note sp-core does include substrate-bip39 (optional, yet included via the std feature) and this is accordingly a tightly bound library.

Apologies if I should've made the issue under polkadot-sdk to adopt this repo, instead of making the issue here to be moved.

[nuke-web3]

Seems reasonable to me, the move seems to be more centralized codebase vs. more independent crates/modules. I do think this repo is essentially unmonitored, thus an issue on the SDK is likely to get more eyes. Linking to this one is probably best to crossref IMHO

[andresilva]

Done in https://github.com/paritytech/polkadot-sdk/pull/3579.