types(models+query): infer return type from schema for 1-level deep nested paths

[vkarpov15]

Fix #14615
Re: #13836
Re: #12064

Summary

Fixes #14615 by inferring the type of 1-level deep schema paths to help with query filters. Consider the following schema:

  const FooSchema = new Schema({
    one: { type: String }
  });

  const MyRecordSchema = new Schema({
    _id: { type: String },
    foo: { type: FooSchema }
  });

Currently, Mongoose can infer await MyRecord.distinct('foo').exec() return type because foo is a top-level path. But not await MyRecord.distinct('foo.one').exec() because foo.one is nested.

As per discussion in #12064, we don't want to infer arbitrarily deep nested paths: too risky because we don't want to run into the dreaded Type instantiation is excessively deep and possibly infinite error in an unforeseen case, and we don't want to cause performance issues.

However, #14615 encourages us to consider just the case of 1-level deep nested paths. If we cut off at 1 level, we don't need to worry about infinite recursion and risk of performance degradation is much lower.

This PR adds a reasonably simple WithLevel1NestedPaths helper which, given a type, also adds all the 1-level deep nested paths to the result type. That makes it easy to get the correct result type for distinct() with 1-level deep nested paths. This PR intentionally avoids arrays.

Examples

[hasezoey]

LGTM

[vkarpov15]

Merging this into 8.5 branch in the interest of caution