Add Identifier section to Core Profile

[benfrancis]

Previously suggested in #73 (comment) and #78

I propose adding a section to the WoT Core Profile section called "Identifier" which defines the URI which identifies the Core Profile, to be used as a value of the profile member of Thing Descriptions to denote that a given Thing Description conforms to the Core Profile.

In examples I have so far used the URI https://www.w3.org/2022/wot/profile/core. How does everyone feel about that URI? Do we need to add a version number?

[benfrancis]

I have submitted a pull request (#117) which adds an identifier sub-section to Core Profile and uses the identifier https://www.w3.org/2022/wot/profile/core/v1