add(zig-interface): dependency to check interface contracts at comptime

The described interfaces for `Widget` and `Layout` are now defined and
correspondingly checked at comptime.

build.zig

@@ -15,13 +15,21 @@ pub fn build(b: *std.Build) void {
     // set a preferred release mode, allowing the user to decide how to optimize.
     const optimize = b.standardOptimizeOption(.{});
 
-    const zg = b.dependency("zg", .{});
+    const zg = b.dependency("zg", .{
+        .target = target,
+        .optimize = optimize,
+    });
+    const interface = b.dependency("interface", .{
+        .target = target,
+        .optimize = optimize,
+    });
 
     const lib = b.addModule("zterm", .{
         .root_source_file = b.path("src/zterm.zig"),
         .target = target,
         .optimize = optimize,
     });
+    lib.addImport("interface", interface.module("interface"));
     lib.addImport("code_point", zg.module("code_point"));
 
     const exe = b.addExecutable(.{