Implement shift-select config as a built-in editor feature

Currently there are 2 ways of selecting blocks of text. The old, modal way of entering select mode and using movement commands and the newer, GUI-style way of holding down Shift with movement keys.

The latter is implemented as a built-in config file, which was originally just a quick experiment to see if it'd work. After using it for several months, I'm sure that it works well, but there are a few problem preventing it from being enabled by default. The first is that it inherently conflicts with the old, modal select mode -- it's only possible to use one or the other, not both. The second is that not every terminal supports Shift modified keys, so enabling the shift-select bindings by default would leave users of less modern terminals without any way to make selections.

The only way to allow Shift selection key bindings in the default config while also providing the modal select mode as a fallback option is to properly integrate both into the editor as built-in features, instead of as key binding hacks.