Add WASM-specific configurations

I've added a runner override to use the wasm-server-runner for easy
devlopment -- it's a localhost webserver to provide the WASM binary &
it's bootstrap code.

There is also target-specific config related to the RNG back-end.

.cargo/config.toml

@@ -0,0 +1,4 @@
+
+[target.wasm32-unknown-unknown]
+runner = "wasm-server-runner"
+rustflags = ["--cfg", "getrandom_backend=\"wasm_js\""]