check that schema type is an object when setting isUnderneathDocArray

[vmo-khanus]

Summary
Originally mentioned here:
#10193 which was (at least partially for us) fixed by @vkarpov15

When checking if isUnderneathDocArray should be true for a given schema the root object of a sub document can be just 'nested' and so it throws an error: "Cannot create property '$isUnderneathDocArray' on string 'nested'".

This fix just slightly extends the work of @vkarpov15 by adding his fix to schemaType.schema.paths and schemaType.schema.subpaths in addition to schemaType.schema.singleNestedPaths.

More recently mentioned here:
#10357

Examples
Slightly modified version @IslandRhythms example (removed need to connect to db since this happens on schema compilation):

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const OutputSchema = new Schema({
      1: { left: { type: String }},
});

const AslSchema = new Schema({
  output: { type: OutputSchema }
});

const SessionSchema = new Schema({
  asls: { type: [AslSchema] }
});

const BlockSchema = new Schema({
  sessions: { type: [SessionSchema]  }
});

const Test = mongoose.model('Test', BlockSchema);

async function test() {
    await Test.validate({sessions: {asls: [{output: {1: {left: 'Test'}}}]}});
}

test();