Add WASM target to the nix flake

[FireFragment]

This makes trunk serve working out of the box through Nix and the nix develop command.

Before:

error[E0463]: can't find crate for `core`
  |
  = note: the `wasm32-unknown-unknown` target may not be installed
  = help: consider downloading the target with `rustup target add wasm32-unknown-unknown`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `ttf-parser` (lib) due to 1 previous error