fix: array schema definitions with of keyword

[IslandRhythms]

closes #14416

[vkarpov15]

LGTM. Merging into 8.3 as a new feature since I thought we already supported type: Array, of: Number

[forivall]

Could remove these comments now.

Suggested change

	assert.equal(schema.path('nums.$').caster.instance, 'Number'); // actually Mixed
	assert.equal(schema.path('tags.$').caster.instance, 'String'); // actually Mixed
	assert.equal(schema.path('subdocs.$').casterConstructor.schema.path('name').instance, 'String'); // actually Mixed
	assert.equal(schema.path('nums.$').caster.instance, 'Number');
	assert.equal(schema.path('tags.$').caster.instance, 'String');
	assert.equal(schema.path('subdocs.$').casterConstructor.schema.path('name').instance, 'String');