Function GetPlaylistItems uses the wrong endpoint

[Lanver33]

The name of this function is misleading since it uses the deprecated endpoint "playlists/%s/tracks"

func (c *Client) GetPlaylistItems(ctx context.Context, playlistID ID, opts ...RequestOption) (*PlaylistItemPage, error) {
	spotifyURL := fmt.Sprintf("%splaylists/%s/tracks", c.baseURL, playlistID)
        ...
}

Instead of the actual endpoint that the API expects "playlists/%s/items" https://developer.spotify.com/documentation/web-api/reference/get-playlists-items

func (c *Client) GetPlaylistItems(ctx context.Context, playlistID ID, opts ...RequestOption) (*PlaylistItemPage, error) {
	spotifyURL := fmt.Sprintf("%splaylists/%s/items", c.baseURL, playlistID)
        ...
}

[Lanver33]

I wrote a fix in this pull request: #294

[jdcukier]

#296 has a pretty thorough fix