Mechanism to rename config fields and retain backwards compatibility

[mitchellh]

Now that we're a public project I'm more sensitive to maintaining backwards compatibility. I don't have any official policy around it [yet] except to "try to retain it unless necessary to break."

We now have a concrete use case for renaming a config field in #4403. We want to rename background-blur-radius to background-blur.

To do this, I want to introduce a mechanism to src/cli/args.zig so that we can rename fields and have the old one map to the new one.

[jcollie]

I think that a comptime-generated struct pub fn Deprecated(comptime renamed_to: []const u8) type could do the job.

[pluiedev]

Maybe we should instead have versioned config migrations that allow us to have old fields with old types that can be converted into new fields with new types (and chain them all together, of course)? This way we can more robustly support older versions when the migration steps required involve more than just a simple renaming. Doing this in other languages would be really annoying but with Zig comptime I think it's a lot more managable

/// Config as in 1.0.1
pub const ConfigV1_0_1 = {
	// fields
    @"background-blur-radius": u8 = 0,
    @"adw-toast": AdwToast = ...,
    @"hypothetical-field-to-remove": u32 = 0,
};

/// Config as in 1.1.0
pub const ConfigV1_1_0 = Migration(
    // keys to remove
    &.{
        .@"background-blur-radius",
        .@"adw-toast",
        .@"hypothetical-field-to-remove",
    }, 
    
    // keys to add
    struct {
        @"background-blur": BackgroundBlur = .false,
        @"app-notifications": AppNotifications = .{ .clipboard_copy = true },
        @"sparkling-new-field": ?[]const u8 = null,
    },
    
    struct {
        // Old will be a subset of fields matching the keys specified in first argument
        fn migrate(old: ConfigV1_1_0.Old) ConfigV1_1_0.New {
            return .{
                .@"background-blur" = .{ .radius = old.@"background-blur-radius" },
                .@"app-notifications" = old.@"adw-toast",
                .@"sparkling-new-field" = null,
            };
        }
    }.migrate,
);