Make context menu customizable

[abichinger]

This PR adds support for custom context menu items.

Overview of changes:

• Added prop called contextMenuItems
  https://github.com/abichinger/vuefinder/blob/0dea1a006028c55a24286a2b9a1f9dae92cfbc49/src/components/VueFinder.vue#L124
• New library exports: contextMenuItems and SimpleContextMenuItem
  https://github.com/abichinger/vuefinder/blob/0dea1a006028c55a24286a2b9a1f9dae92cfbc49/src/index.js#L24
• New example called: Custom context menu example
  https://github.com/abichinger/vuefinder/blob/0dea1a006028c55a24286a2b9a1f9dae92cfbc49/examples/App.vue#L52
• This is the type of an item:

type ContextMenuItem = {
    title: (i18n: App["i18n"]) => string;
    action: (app: App, selectedItems: DirEntry[]) => void;
    link?: (app: App, selectedItems: DirEntry[]) => void;
    show: (app: App, ctx: MenuContext) => boolean;
}

[n1crack]

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