[dev.typealias] spec: use term 'embedded field' rather than 'anonymous field'

First steps towards defining type aliases in the spec.
This is a nomenclature clarification, not a language change.

The spec used all three terms 'embedded type', 'anonymous field',
and 'embedded field'. Users where using the terms inconsistently.

The notion of an 'anonymous' field was always misleading since they
always had a de-facto name. With type aliases that name becomes even
more important because we may have different names for the same type.

Use the term 'embedded field' consistently and remove competing
terminology.

For #18130.

Change-Id: I2083bbc85788cab0b2e2cb1ff58b2f979491f001
Reviewed-on: https://go-review.googlesource.com/35108


Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>