feat(child_process): add `spawn`

src/child_process_ffi.mjs

···
+import child_process from "node:child_process"
+
+export const spawn = (cmd, args) =>
+  child_process.spawn(cmd, args.toArray())

src/plinth/node/child_process.gleam

···
+pub type ChildProcess {
+  ChildProcess(kill: fn() -> Nil, stdin: Stdin)
+}
+
+pub type Stdin {
+  Stdin(write: fn(String) -> Nil)
+}
+
 @external(javascript, "child_process", "exec")
-pub fn exec(command: String) -> Nil
+pub fn exec(command: String) -> ChildProcess
+
+@external(javascript, "../../child_process_ffi.mjs", "spawn")
+pub fn spawn(command: String, arguments: List(String)) -> ChildProcess