Better checking of assignment declarations

[sandersn]

Previously, type checking was turned off for all assignment declarations. This is a problem when the declarations are annotated with jsdoc types:

/** @type {string} */
module.exports = 0

As @DanielRosenwasser points out, this will become more common as people use checkJS more and more.

This PR checks assignment declarations, except for expando initialisers. Expando initialisers are

1. Empty object types.
2. Function types.
3. Class types.
4. Non-empty object types when the assignment declaration kind is
   prototype assignment or module.exports assignment.

It's probably possible to write a reasonable ad-hoc check for expando initalisers too, but they are (I think) least likely to be annotated since it looks weird:

/** @type { x: number, y: string } */
var ns = {}
ns.x = 1
ns.y = 'oh no'

Fixes #27327