Previously,  `this: this` in constructor functions only when there was
an explicit `@constructor` tag on the function. Now, `this: this` for
any function that's known to be a constructor function.

This improves completions inside constructor functions; also note that
previously the compiler *did* type `this: this` inside methods of constructor
functions, so this fix makes us more consistent. This is reflected in
the large number of baselines that improve.

The fix is a simple switch to `isJSConstructor`, which is the standard
way to detect constructor functions. I'm not sure why the original PR
didn't use this method.

I remember discussing this limitation in the original bug, #25979, and
I guess I decided that it made sense. But I was heavily primed by the bug's
framing of the problem in terms of `noImplicitThis`, which *should*
require an explicit `@constructor` tag.

With better typing comes better detection of `@readonly` assignment; I
had to fix the readonly detection code to use `isJSConstructor` as well.

The new rules mean that it never applies. It's possible that it should
apply to functions like

```js
function f() {
  this.init()
}
```

In which `init` is never defined, but I think this program is incomplete
enough that not offering the fix is fine.

Previously, both `@class` and `@this` in a jsdoc would cause the `@this`
annotation to be ignored. This became a worse problem with this PR,
because `this` is correctly typed even without the annotation.

This commit makes sure that `@this` is checked first and used if
present.