mach: require a 'pub const App' to be exposed

This requires apps expose a `pub const App` from their `main.zig`, effectively adopting
the high-level/low-level application API outlined in hexops/mach#349

If `pub const App` does not exist, a clear compiler error is produced:

```
./src/platform/common.zig:5:9: error: expected e.g. `pub const App = mach.App(modules, init)' (App definition missing in your main Zig file)
        @compileError("expected e.g. `pub const App = mach.App(modules, init)' (App definition missing in your main Zig file)");
        ^
./src/platform/native.zig:13:28: note: called from here
    common.checkApplication(app_pkg);
                           ^
./src/platform/native.zig:12:10: note: called from here
comptime {
         ^
./src/platform/native.zig:15:20: error: container '.app' has no member called 'App'
const App = app_pkg.App;
                   ^
```

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

src/platform/common.zig

@@ -1,6 +1,11 @@
 const Core = @import("../Core.zig");
 
-pub fn checkApplication(comptime App: type) void {
+pub fn checkApplication(comptime app_pkg: type) void {
+    if (!@hasDecl(app_pkg, "App")) {
+        @compileError("expected e.g. `pub const App = mach.App(modules, init)' (App definition missing in your main Zig file)");
+    }
+    const App = app_pkg.App;
+
     if (@hasDecl(App, "init")) {
         const InitFn = @TypeOf(@field(App, "init"));
         if (InitFn != fn (app: *App, core: *Core) @typeInfo(@typeInfo(InitFn).Fn.return_type.?).ErrorUnion.error_set!void)

src/platform/native.zig

@@ -1,13 +1,19 @@
 const std = @import("std");
 const glfw = @import("glfw");
 const gpu = @import("gpu");
-const App = @import("app");
+const app_pkg = @import("app");
 const Core = @import("../Core.zig");
 const structs = @import("../structs.zig");
 const enums = @import("../enums.zig");
 const util = @import("util.zig");
 const c = @import("c.zig").c;
 
+const common = @import("common.zig");
+comptime {
+    common.checkApplication(app_pkg);
+}
+const App = app_pkg.App;
+
 pub const scope_levels = if (@hasDecl(App, "scope_levels")) App.scope_levels else [0]std.log.ScopeLevel{};
 pub const log_level = if (@hasDecl(App, "log_level")) App.log_level else std.log.default_level;
 
@@ -581,11 +587,6 @@ pub const Platform = struct {
 
 pub const BackingTimer = std.time.Timer;
 
-const common = @import("common.zig");
-comptime {
-    common.checkApplication(App);
-}
-
 pub fn main() !void {
     var gpa = std.heap.GeneralPurposeAllocator(.{}){};
     defer _ = gpa.deinit();

src/platform/wasm.zig

@@ -1,5 +1,5 @@
 const std = @import("std");
-const App = @import("app");
+const app_pkg = @import("app");
 const Core = @import("../Core.zig");
 const structs = @import("../structs.zig");
 const enums = @import("../enums.zig");
@@ -29,6 +29,12 @@ const js = struct {
     extern fn machPanic(str: [*]const u8, len: u32) void;
 };
 
+const common = @import("common.zig");
+comptime {
+    common.checkApplication(app_pkg);
+}
+const App = app_pkg.App;
+
 pub const CanvasId = u32;
 
 pub const Platform = struct {
@@ -273,11 +279,6 @@ pub const BackingTimer = struct {
     }
 };
 
-const common = @import("common.zig");
-comptime {
-    common.checkApplication(App);
-}
-
 var app: App = undefined;
 var core: Core = undefined;