Weird limitations on 3d mouse navigation schemes in `Editor Settings/Editors/3D/Navigation`

### Tested versions

4.6 and 4.5 for sure, but as far back as I can recall in the 4.x series.

### System information

Godot v4.6.stable - Linux Mint 22.1 (Xia) on X11 - X11 display driver, Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 2060 (nvidia; 590.48.01) - AMD Ryzen 9 5900XT 16-Core Processor (32 threads) - 62.70 GiB memory

### Issue description

I have 12 years of muscle memory from doing CAD in onshape, which uses:
```
Orbit mouse button: right mouse
Pan mouse button: middle mouse
```
-- both WITHOUT modifiers.

I cannot replicate this setup with any of the presets, nor with a "custom" setup. All of the presets and custom settings REQUIRE different combinations of modifiers, even if though they let you change which mouse button you can use unmodified (even the right mouse button can be used, so this limitation doesn't seem to be about prevent collisions with other mouse usages in the editor), and even though the different presets use a range of different modifier keys (so this limitation doesn't seem to be about preventing collisions with mouse+mod combos in the editor).

It would be amazing to be able to independently select the button+kbd mod combo when setting up a custom scheme. For those of us coming from other programs, the benefit of this little bit of configurability is huge! Thanks!! :-)

### Steps to reproduce

just navigate in the menus to `Editor Settings/Editors/3D/Navigation` and try playing with the options.

### Minimal reproduction project (MRP)

MRP not required, this is a settings issue not a project issue; will show up with any project.

Start in Editor Settings/Editors/3D/Navigation and reproduce the limitation by trying the presets and custom options with unmodified mouse buttons. Done means a custom navigation scheme can independently select the mouse button and keyboard modifier for orbit and pan, including the requested unmodified right-button orbit and middle-button pan setup.

Written by the indexing model from the issue text.