examples: move core examples into core/ folder; add triangle example

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

examples/core/triangle/main.zig

@@ -0,0 +1,15 @@
+const mach = @import("mach");
+
+// The global list of Mach modules registered for use in our application.
+pub const modules = .{
+    mach.Core,
+    @import("App.zig"),
+};
+
+pub fn main() !void {
+    // Initialize mach.Core
+    try mach.core.initModule();
+
+    // Main loop
+    while (try mach.core.tick()) {}
+}