Validate MSC4426 User Status profile fields

Currently we have no validation on Synapse side for accepting MSC4426 user status profile fields from clients. The MSC clearly notes that some validation SHOULD be done by homeservers. Given these fields are something Element Web and Element X clients already create and consume, we should help them by introducing the validation in the (albeit work in progress) MSC.

See the details of the MSC field grammar at: https://github.com/matrix-org/matrix-spec-proposals/blob/anoa/user_status_profile_fields/proposals/4426-user-status-profile-fields.md#text-and-emoji-field-grammar