Add ability to listen (tap) the input

By doing something like
```
fn input_output_loop(input_output: Subject(List(String))) -> Nil {
  let output = process.receive_forever(input_output)

  echo output

  input_output_loop(input_output)
}
```

src/input/key.gleam

@@ -1,4 +1,5 @@
 import gleam/erlang/atom
+import gleam/erlang/process.{type Subject}
 import gleam/list
 import gleam/string
 
@@ -57,14 +58,20 @@ pub fn start_raw_shell() {
   internal_input.shell_start_interactive(#(no_shell, raw))
 }
 
-pub fn read_input_until_key(l: List(String)) -> Key {
+pub fn read_input_until_key(
+  l: List(String),
+  tap_input: Subject(List(String)),
+) -> Key {
   case
     internal_input.read_input()
     |> list.wrap
     |> list.append(l, _)
     |> from_list
   {
-    Continue(l) -> read_input_until_key(l)
+    Continue(l) -> {
+      process.send(tap_input, l)
+      read_input_until_key(l, tap_input)
+    }
     k -> k
   }
 }

src/mpv/mpv.gleam

@@ -9,8 +9,13 @@ import mpv/control.{type Control}
 import tcp/reason
 import tcp/tcp.{type Socket}
 
-type State(socket, inject_input, exit) {
-  State(socket: Socket, inject_input: Subject(Key), exit: Subject(Nil))
+type State(socket, inject_input, tap_input, exit) {
+  State(
+    socket: Socket,
+    inject_input: Subject(Key),
+    tap_input: Subject(List(String)),
+    exit: Subject(Nil),
+  )
 }
 
 pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
@@ -26,8 +31,11 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
       let inject_input_name = process.new_name("inject_input")
       let inject_input = process.named_subject(inject_input_name)
 
+      let tap_input_name = process.new_name("tap_input")
+      let tap_input = process.named_subject(tap_input_name)
+
       case
-        actor.new(State(socket, inject_input, exit))
+        actor.new(State(socket, inject_input, tap_input, exit))
         |> actor.on_message(handle_message)
         |> actor.start
       {
@@ -41,7 +49,13 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
             let assert Ok(_) =
               process.register(process.self(), inject_input_name)
 
-            read_input(data, inject_input)
+            read_input(data, inject_input, tap_input)
+          })
+
+          process.spawn(fn() {
+            let assert Ok(_) = process.register(process.self(), tap_input_name)
+
+            input_output_loop(tap_input)
           })
 
           Ok(Nil)
@@ -52,9 +66,9 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
 }
 
 fn handle_message(
-  state: State(socket, inject, exit),
+  state: State(socket, inject, input_output, exit),
   control: Control,
-) -> actor.Next(State(socket, inject, exit), Control) {
+) -> actor.Next(State(socket, inject, input_output, exit), Control) {
   case control {
     control.TogglePlayPause -> {
       echo "toggling play/pause"
@@ -83,16 +97,20 @@ fn handle_message(
 /// messages to `inject_input` which will initialize the "input to key" sequence.
 /// This is useful to ultimately create a `Control` without the user having to
 /// input all of the character(s) needed.
-fn read_input(subject: Subject(Control), inject_input: Subject(Key)) -> Nil {
+fn read_input(
+  subject: Subject(Control),
+  inject_input: Subject(Key),
+  tap_input: Subject(List(String)),
+) -> Nil {
   let buffer = case process.receive(inject_input, 1) {
     Ok(key.Continue(buffer)) -> buffer
     Ok(_) | Error(_) -> []
   }
 
   let _ =
-    key.read_input_until_key(buffer)
+    key.read_input_until_key(buffer, tap_input)
     |> control.from_key
     |> result.map(process.send(subject, _))
 
-  read_input(subject, inject_input)
+  read_input(subject, inject_input, tap_input)
 }