add(main): main executable with zig build run support

The main executable contains a simple `Element` implementation example
showing how interaction through and outside of the event loop can be
implemented to impact the rendered contents.

build.zig

@@ -17,6 +17,19 @@ pub fn build(b: *std.Build) void {
     });
     lib.addImport("code_point", zg.module("code_point"));
 
+    // main executable (usually used for testing)
+    const exe = b.addExecutable(.{
+        .name = "zterm",
+        .root_source_file = b.path("src/main.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+    exe.root_module.addImport("zterm", lib);
+
+    // TODO: add example execution through optional argument to `zig run` to run
+    // an example application instead of the main executable
+
+    // example applications (usually used for documentation and demonstrations)
     const container = b.addExecutable(.{
         .name = "container",
         .root_source_file = b.path("examples/container.zig"),
@@ -26,7 +39,19 @@ pub fn build(b: *std.Build) void {
     container.root_module.addImport("zterm", lib);
     b.installArtifact(container);
 
-    // testing
+    // zig build run
+    const run_cmd = b.addRunArtifact(exe);
+    run_cmd.step.dependOn(b.getInstallStep());
+    // Allow additional arguments, like this: `zig build run -- arg1 arg2 etc`
+    if (b.args) |args| run_cmd.addArgs(args);
+
+    // This creates a build step. It will be visible in the `zig build --help` menu,
+    // and can be selected like this: `zig build run`
+    // This will evaluate the `run` step rather than the default, which is "install".
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+
+    // zig build test
     const lib_unit_tests = b.addTest(.{
         .root_source_file = b.path("src/zterm.zig"),
         .target = target,