prd: Deleting a relation using shortcut

#**Is your feature request related to a problem? Please describe.**
When I work with text annotations and I want to delete an annotation is annoying to always click on the garbage icon on the far left.

**Describe the solution you'd like**
Ideal would be some kind of shortcut. My initial idea was "delete" button. So, I would first click on a relation to highlight it and then press "delete" to delete the relation.

**Describe alternatives you've considered**
None

**Additional context**
I already tried to implement this but I am fairly new to coding so I would appreciate all help. Will create a PR if this gets approved (hopefully I understood the process right).

**PoC**
Here is what I implemented: [https://github.com/m4tej241/label-studio/tree/syncWithRelationsTab](url)