…argeting Safari <=v15

The Angular compiler is dependent on static fields being attached to
user-defined classes. e.g. `static ecmp = defineComponent`.

These static fields sometimes rely on variables from outside of the
class. e.g. the Angular compiler generates constants for content
projection that are then accessed in the static field initializer.

Surprisingly such access to these variables may break in Safari <=v15
when a page is loaded without devtools open. The bug (already solved in
v16 of Safari)- is very subtle, hard to re-reproduce but basically
variable scope tracking is broken. This bug is triggered by additional
parenthesis in the initializer expression. See:
https://bugs.webkit.org/show_bug.cgi?id=236843.

The TypeScript compiler may generate such additional parenthesis when
it tries to adjust the `this` context when invoking methods, such as for
defining animations in the `ecmp` definition.

More details can be found here:
#24355 (comment)

To ensure Angular applications are not subject to this bug when
targeting Safari <=v15. v15 Safari, both for iOS and Mac is still part of
the default CLI browserslist with `last 2 Safari majors` (at time of
writing).

Note that it is important that the Babel plugin properly handles the
downleveling of static block-defined members. TypeScript will transform
static fields, like `static ecmp` into `static { this.ecmp = X }` when
`useDefineForClassFields = false` (which is the case for CLI apps). The
class properties plugin from Babel seems to handle this in an acceptable
way. Unlike actual static fields, Babel will not use helpers like
`defineProperty` for such extracted static blocks though. e.g.

See repro: https://gist.github.com/devversion/dec0dea26e348c509921bf62079b60be

```js
class Test {
  x = true;

  static b = true;
  static {
    this.a = true;
  }
}

// into

class X {
  constructor() {
    _defineProperty(this, "x", true);
  }
}
_defineProperty(X, "b", true);
X.a = true;
```

note that in practice TypeScript with `useDefineForClassFields = false`
will put non-static members into the constructor as normal assignments
regardless- so there would be no change by the Babel plugin.

Fixes #24355.