text selection

[felixangell]

text should be select-able with shift arrow combo. this should create a marker on the text which creates a selection. actions can be performed on the selections.

[felixangell]

tracking:

• [ ] make it so that the current selection is invalidated e.g. when we remove shift key and move?
• [ ] fix panic when we try select out of the file bounds
• [ ] pretty-ify select rendering (outline box, rounded corners?)
• [ ] colour config for selection boxes
• [ ] copy selection
• [ ] ...