Extract mpv/key to input

To separate the concern from `mpv`

src/input/internal.gleam

@@ -0,0 +1,15 @@
+import gleam/erlang/atom
+
+pub fn read_input() -> String {
+  io_get_chars("", 1)
+}
+
+pub type NotUsed
+
+// https://www.erlang.org/doc/apps/stdlib/shell.html#start_interactive/1
+@external(erlang, "shell", "start_interactive")
+pub fn shell_start_interactive(options: #(atom.Atom, atom.Atom)) -> NotUsed
+
+// https://www.erlang.org/doc/apps/stdlib/io.html#get_line/1
+@external(erlang, "io", "get_chars")
+fn io_get_chars(prompt: String, count: Int) -> String