ngModuleDef is a definition field for @NgModule, which passes module
metadata through to the runtime. This metadata is broadly divided into two
categories: metadata required for the runtime in all cases, and metadata
only required to support JIT compilation usage of the module.

There are two things about this JIT metadata that make it special:

1) it's not needed in AOT-compiled apps (the vast majority of apps)
2) it breaks tree-shaking to include, as it creates hard references from
the root NgModule to every declared/imported module/component/directive/pipe
in the app, regardless of whether they're actually used in the template.

To support JIT apps, this metadata must be included in generated code,
especially if that code is shipped to NPM. Therefore, to avoid breaking tree
shaking it must be _removed_ in AOT-only apps.

Previously, the metadata was patched onto the ngModuleDef via a special
setNgModuleScope call following the class, which was marked as "pure". Thus,
an optimizer like Terser would remove the call when optimizing a production
bundle.

This has a significant downside: it breaks production optimization of JIT
applications, which do need this metadata call to survive for runtime. JIT
optimization should not be coupled to the rest of tree shaking.

Instead, this commit introduces a compile-time constant `ngJitMode`. A
second argument to defineNgModule carries the JIT part of the definition,
which is guarded with `ngJitMode`:

```typescript
class FooModule {}
FooModule.ngModuleDef = defineNgModule(
  // The main module definition, included in all builds:
  {type: FooModule, id: 'foo'},
  // The JIT module definition, guarded by a compile-time constant:
  ngJitMode && {declarations: [FooCmp]},
);
```

Terser can be configured to define ngJitMode as false, which will result in
the JIT part of the definition shaking away. A suitable configuration is
provided in tooling.ts for use in the CLI.