Universal feature flags (compile-time and runtime)

[max-privatevoid]

I want to make optional features fully optional. Of course anything that's not "core functionality" will be runtime-disabled by default, but it would also be nice to be able to produce custom-tailored builds of Zedless that don't even include the code for that functionality. This will be especially useful for features that require large dependencies.

There should be

• A central place in the codebase to define feature flags.
• A way to select which features to include at build-time (cargo features and some mechanism to select them with Nix).
• A runtime setting that allows users to enable those features.
• A section on the welcome screen that shows all built-in features and allows users to enable them, similar to how Vim mode is handled currently.