feat(lib): add playback rate config options (#320)

* feat(lib): add playback rate config options

Basic playback rate config options, closes #309

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

manim_slides/slide/base.py

@@ -255,7 +255,13 @@ class BaseSlide:
         self._current_animation += 1
 
     def next_slide(
-        self, *, loop: bool = False, auto_next: bool = False, **kwargs: Any
+        self,
+        *,
+        loop: bool = False,
+        auto_next: bool = False,
+        playback_rate: float = 1.0,
+        reversed_playback_rate: float = 1.0,
+        **kwargs: Any,
     ) -> None:
         """
         Create a new slide with previous animations, and setup options
@@ -276,6 +282,14 @@ class BaseSlide:
 
             Note that this is only supported by ``manim-slides present``
             and ``manim-slides convert --to=html``.
+        :param playback_rate:
+            Playback rate at which the video is played.
+
+            Note that this is only supported by ``manim-slides present``.
+        :param reversed_playback_rate:
+            Playback rate at which the reversed video is played.
+
+            Note that this is only supported by ``manim-slides present``.
         :param kwargs:
             Keyword arguments to be passed to
             :meth:`Scene.next_section<manim.scene.scene.Scene.next_section>`,
@@ -375,7 +389,12 @@ class BaseSlide:
 
             self._current_slide += 1
 
-        self._pre_slide_config_kwargs = dict(loop=loop, auto_next=auto_next)
+        self._pre_slide_config_kwargs = dict(
+            loop=loop,
+            auto_next=auto_next,
+            playback_rate=playback_rate,
+            reversed_playback_rate=reversed_playback_rate,
+        )
         self._start_animation = self._current_animation
 
     def _add_last_slide(self) -> None: