Improve toggling and clearing debugger breakpoints

We have debugger support since a few months, and it works, but the UX could use some more refinement.

Clearing breakpoints is tedious at the moment at you have to go to every line with a breakpoint and run the remove breakpoint command.

Here are some tasks to improve the UX.

## New commands

- [ ] Remove breakpoints in range: loop over all breakpoints in the selected range and remove them.
- [ ] Remove breakpoints in buffer: using the range function above, remove breakpoints in the entire buffer.
- [ ] Remove breakpoints in defun. Same as above but for the defun where the point is, so you don't have to do a selection first. GDScript code is mostly functions (methods) so that could come in handy.

## Usability

- [x] Add shortcut for toggle breakpoint.
- [x] Remove shortcuts for add and remove breakpoints, they'll be replaced by the above.
- [x] Add shortcuts that match Godot's debugger and run? F5 to F9 are not bound by default.