feat: add invideoquiz xblock

[alenkadev]

Description

Adds a new MFE-based Studio editor for the In-Video Quiz (invideoquiz) XBlock, replacing the legacy raw XML/JSON editing experience with a proper form-based UI inside frontend-app-authoring.

Course Authors can now, from a unit page:

• Select which video in the unit the quiz is attached to
• Add one or more timed quiz entries, each pairing a video timestamp with a problem from the unit
• Configure an optional "jump back" time per entry
• Save directly through the standard Studio editor Save flow

Specific changes included:

• New InVideoQuizEditor component, Redux slice (inVideoQuiz), and thunk actions for loading/saving quiz configuration (selectedVideo, quizItems) against the XBlock's backend fields.
• Registered invideoquiz in supportedEditors.ts so it opens the new editor instead of the legacy one.
• Editor now automatically opens immediately after a new In-Video Quiz component is added to a unit, instead of requiring a separate click to edit.
• Save button now reads "Save" (previously "Add to course").
• A dismissible "Content not found" alert now shows in the editor if the containing unit has no video or no problem components yet, since the quiz can't be configured without both.
• Single-select and dropdown problem types now enforce exactly one correct answer — selecting a new correct answer automatically clears the previous selection, instead of allowing multiple.

Who this impacts:

• Course Author — primary audience; this is the whole editing experience for the block.
• Developer — introduces a new editor pattern/Redux module under src/editors/ for anyone extending or maintaining this XBlock's authoring UI.

Supporting information

Corresponds to internal ticket LP-895 and companion work on the openedx/xblock-in-video-quiz package (v2.0.0), which this editor's saved fields (timemap, jump_back, video_id) map onto.