Simplify input capture

Instead of "injecting" characters into the input stream, the input
stream is now forwarded to the `musicplayer`. It has will have to
decide what to do with the stream, e.g. by setting the "mode" to
something that captures the input stream and acts upon it

test/musicplayer/input/input_test.gleam

@@ -1,7 +1,7 @@
 import gleam/list
 import gleeunit
 
-import musicplayer/input/key.{type Key, Char, csi, esc, input_introducer as ii}
+import musicplayer/input/key.{type Key, Char, backspace, csi, esc, return}
 
 pub fn main() -> Nil {
   gleeunit.main()
@@ -27,11 +27,8 @@ pub fn key_from_list_test() {
   ]
 
   let input_tests = [
-    TestCase([ii], key.Continue([ii])),
-    TestCase([ii, "a"], key.Continue([ii, "a"])),
-    TestCase([ii, "a", "b"], key.Continue([ii, "ab"])),
-    TestCase([ii, "ab", "\u{007F}"], key.Continue([ii, "a"])),
-    TestCase([ii, "ab", "\r"], key.Input("ab")),
+    TestCase([return], key.Return),
+    TestCase([backspace], key.Backspace),
   ]
 
   let test_cases = [base_tests, char_tests, escape_tests, input_tests]