Add support for `wasm32-wasi` target

[andrewvious]

Working on a project that uses said target to run, and I'm having to use rustup everytime. Would be useful if the wasm32-wasi target were available in the supported targets.

[rvolosatovs]

@andrewvious the wasm32-wasi is already supported by this flake for a while, I've been using it over a year now on x86_64 and aarch64 Linux and Darwin with no issues.

For example:

$ nix build github:nix-community/fenix#targets.wasm32-wasi.latest.rust-std
$ tree ./result
./result
└── lib
    └── rustlib
        └── wasm32-wasi
            └── lib
                ├── libaddr2line-9af6b5f3e66efc03.rlib
                ├── libadler-09d6a21c31cdc333.rlib
                ├── liballoc-c11c740ac1bfd7ae.rlib
                ├── libcfg_if-d38771e9de33815c.rlib
                ├── libcompiler_builtins-bbcf3577cf73f757.rlib
                ├── libcore-9646fcd877534fb2.rlib
                ├── libgetopts-a96c6a90fd634e49.rlib
                ├── libgimli-9a04fdba4c6ec6dd.rlib
                ├── libhashbrown-157a3a9deae9b2bb.rlib
                ├── liblibc-bab80dc78973e194.rlib
                ├── libmemchr-552b6f8f79141d91.rlib
                ├── libminiz_oxide-16aeecf3b3629014.rlib
                ├── libobject-fd5008d0bcfd392e.rlib
                ├── libpanic_abort-be5777874e9ab206.rlib
                ├── libpanic_unwind-4d56715f9083e70f.rlib
                ├── libproc_macro-5fe356ee003a336c.rlib
                ├── librustc_demangle-4a423cec7e40fe50.rlib
                ├── librustc_std_workspace_alloc-9d5a444aa1b598ea.rlib
                ├── librustc_std_workspace_core-40fe888f92874113.rlib
                ├── librustc_std_workspace_std-0f104b6c5176d1ee.rlib
                ├── libstd-ab602b269ccc5f9e.rlib
                ├── libstd_detect-2cf94704f851bcce.rlib
                ├── libsysroot-fad20c27128402a0.rlib
                ├── libtest-a1d8c7e488f996fe.rlib
                ├── libunicode_width-a9f08909f909fb10.rlib
                ├── libunwind-662c737beb4dc6a8.rlib
                ├── libwasi-768f983517316883.rlib
                └── self-contained
                    ├── crt1-command.o
                    ├── crt1-reactor.o
                    └── libc.a

[andrewvious]

The readme could be updated then I think. I can open a PR to add it to the list of targets.