feat: update next unit button plugin for left sidebar navigation usage (#1578)

* feat: move unit next button slot to plugins folder

* feat: update unit navigation at top to use next unit plugin

* fix: remove 2u plugin specific code

src/plugin-slots/NextUnitTopNavTriggerSlot/README.md

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+# Next Unit Top Navigation Trigger Slot
+
+### Slot ID: `next_unit_top_nav_trigger_slot`
+
+## Description
+
+This slot is used to replace/modify/hide the next button used for unit and sequence navigation at the top of the unit page.
+
+## Example
+
+### Default content
+
+**Next unit button in at top for left sidebar navigation**
+![Screenshot of next unit button slot at the top for left sidebar navigation with default content](./screenshot_unit_at_top_default.png)
+
+**Next unit button in horizontal navigation**
+![Screenshot of horizontal navigaton next unit button slot with default content](./screenshot_horizontal_nav_default.png)
+
+### Replaced with custom component
+
+**Next unit button in at top for left sidebar navigation**
+![Screenshot of 📊 in next unit slot at the top for left sidebar navigation](./screenshot_unit_at_top_custom.png)
+
+**Next unit button in horizontal navigation**
+![Screenshot of 📊 in next unit slot for horizontal navigaton](./screenshot_horizontal_nav_default.png)
+![📊 in next unit slot](./screenshot_horizontal_nav_custom.png)
+
+The following `env.config.jsx` will replace the next unit/sequence button at the top of the unit page. This can be used control the
+button's `onClick` atrribute or change the name of the button.
+
+```js
+import { DIRECT_PLUGIN, PLUGIN_OPERATIONS } from '@openedx/frontend-plugin-framework';
+
+const config = {
+  pluginSlots: {
+    next_unit_top_nav_trigger_slot: {
+      keepDefault: false,
+      plugins: [
+        {
+          op: PLUGIN_OPERATIONS.Insert,
+          widget: {
+            id: 'custom_button_component',
+            type: DIRECT_PLUGIN,
+            RenderWidget: () => (
+              <button>📊</button>
+            ),
+          },
+        },
+      ]
+    }
+  },
+}
+
+export default config;
+```

src/plugin-slots/NextUnitTopNavTriggerSlot/index.tsx

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+import React from 'react';
+
+import { PluginSlot } from '@openedx/frontend-plugin-framework';
+
+import NextButton from '../../courseware/course/sequence/sequence-navigation/generic/NextButton';
+
+interface Props {
+  courseId: string | '';
+  disabled: boolean;
+  buttonText: string | '';
+  nextLink: string;
+  sequenceId: string;
+  unitId: string;
+  onClickHandler: () => void;
+  variant: string;
+  buttonStyle: string;
+}
+
+export const NextUnitTopNavTriggerSlot : React.FC<Props> = ({
+  courseId,
+  disabled,
+  buttonText,
+  nextLink,
+  sequenceId,
+  onClickHandler,
+  variant,
+  buttonStyle,
+}) => (
+  <PluginSlot
+    id="next_unit_top_nav_trigger_slot"
+    pluginProps={{
+      courseId,
+      disabled,
+      buttonText,
+      nextLink,
+      sequenceId,
+      onClickHandler,
+      variant,
+      buttonStyle,
+    }}
+  >
+    <NextButton
+      {...{
+        variant,
+        buttonStyle,
+        onClickHandler,
+        nextLink,
+        disabled,
+        buttonText,
+      }}
+    />
+  </PluginSlot>
+);