fix: drag behaviour for custom vue component

[v1r0x]

Fixes(if relevant):

The drag events don't work with custom vue components in the current version (5.3.2), because event.target is no longer the <a>, but an element inside the custom component. To fix this, I added a function to get the anchor element by going up one level in the DOM until the anchor (class tree-anchor) is found.
I also had to add a check to ondragleave, because the drag event is still fired on the event.target, but this.dropTarget is the anchor (returned by my other function). The function checks if event.target is a child element of this.dropTarget.
There is one lint error left, but I don't know how to fix it. If you could help me again with that, that would be great :)
This patch should fix #7

Checks

• Contains Only One Commit(git reset then git commit)
• Build Success(npm run build)
• Lint Success(npm run lint to check, npm run fix to fix)
• Add Test(if relevant, npm run test to check)
• Add Demo(if relevant)

[plantain-00]

Thanks, it should be available in v5.3.3