Move config into a separate crate

Get miette and knuffel deps contained within.

niri-config/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "niri-config"
+version.workspace = true
+description.workspace = true
+authors.workspace = true
+license.workspace = true
+edition.workspace = true
+repository.workspace = true
+
+[dependencies]
+bitflags.workspace = true
+directories.workspace = true
+knuffel = "3.2.0"
+miette = "5.10.0"
+smithay.workspace = true
+tracing.workspace = true