Grammar and prose are inconsistent about whether undefined is a primitive type

[petervanderbeken]

The grammar has (see https://webidl.spec.whatwg.org/#index-prod-PrimitiveType):

PrimitiveType ::
    UnsignedIntegerType
    UnrestrictedFloatType
    undefined
    boolean
    byte
    octet
    bigint

which seems to imply that undefined is a primitive type.

Primitive types are defined as (see https://webidl.spec.whatwg.org/#dfn-primitive-type): bigint, boolean and the numeric types, which doesn't include undefined.

I think it makes sense to exclude undefined from the primitive types, and the simplest solution would be to move it in the grammar from PrimitiveType to DistinguishableType.