Aligns arrowhead schemas

[steveruizok]

As mentioned in #2516, elements currently store their arrowheads as...

// linear element

{
  ...,
  arrowheadStart: Arrowhead | null,
  arrowheadEnd: Arrowhead | null,
}

...while the appState stores the user's arrowhead selections as:

// app state

{
  ...,
  currentItemArrowheads: {
    start: Arrowhead | null,
    end: Arrowhead | null
  }
}

This PR aligns the two schemas.

In this PR, the app now stores a user's arrowhead selection as appState.currentItemStartArrowhead and appState.currentItemEndArrowhead.

// app state

{
  ...,
  currentItemArrowheadStart: Arrowhead | null,
  currentItemArrowheadEnd: Arrowhead | null,
}

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/excalidraw/excalidraw/r39x57plb
✅ Preview: https://excalidraw-git-align-arrowhead-appstate.excalidraw.vercel.app

[dwelle]

Thanks ❤️