Downloading binaries from `emscripten-asmjs/` and `emscripten-wasm/` dirs in solc-bin

We introduced the new directory structure in solc-bin quite a while ago and we're recommending it in [Static Binaries](https://docs.soliditylang.org/en/latest/installing-solidity.html#static-binaries). solc-js, however, still uses the legacy `bin/` directory. We should switch to the new structure.

- Make `downloadCurrentVersion.ts` use the new dirs.
- It should try `emscripten-wasm/` first and only fall back to `emscripten-asmjs/` when the binary is not there.
- Nightlies are still only available in `bin/`. We need to resolve https://github.com/ethereum/solidity/issues/13015 first.
- Find any other places where we're using the old dirs and fix them too.
- Fix all references to the old dirs in the README.

For reference, Truffle has recently switched to the new structure too: https://github.com/trufflesuite/truffle/pull/5008.

Start with downloadCurrentVersion.ts and search the repository for references to bin/, emscripten-asmjs/, and emscripten-wasm/, including the README. Verify that regular downloads try emscripten-wasm/ before falling back to emscripten-asmjs/, while noting that nightlies remain dependent on issue 13015. Done means other old-directory references are updated without changing the nightly limitation.

Written by the indexing model from the issue text.