Make opposite-shift mode customizable

[bsamseth]

(A potential solution to #1199 and any issues like it)

The current implementation is fine for anyone on an (English) qwerty layout (i.e. almost everybody but a few), but it understandably won't work for non-standard layouts. And even with the standard layout, there are some keys which people can have differing opinions about what is the correct shift key.

As accounting for all kinds of layouts and subjective opinions won't really be feasible, I suggest that we make it configurable. Meaning, in addition to the on/off switch, we make a setting that allows the user to say which keys should go on which side, left or right shift. I would have it defaulted to the current values for qwerty, but editable.

I propose doing something like this for how you set which keys go where (obviously to be made much smarter+prettier, just to give the idea): https://codepen.io/bsamseth/full/wvJpwVr

I'd be happy to make a PR with this if it sounds like something that would be accepted, but I wanted to hear what your thought are before spending any time on it.

As always, hope you are well, and thanks for making a great site!

[Miodec]

Yeah, could add the left and right input fields, and then if they are empty just revert to the default qwerty mode. Just like custom layout fluid