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

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

examples/core-custom-entrypoint/main.zig

@@ -1,15 +0,0 @@
-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()) {}
-}