Grammar for account data types

**Link to problem area**:

https://spec.matrix.org/v1.18/client-server-api/#client-config

**Issue**

No grammar is currently specified for the `type` of account data, so it's unclear for implementations what they have to support.

In particular:
* Which characters are allowed?
* What is the maximum length?
* Is a zero-length type permitted?

It seems like the [common namespaced identifier grammar](https://spec.matrix.org/v1.18/appendices/#common-namespaced-identifier-grammar) would be a good fit.

Start with the client configuration section at the linked Matrix Client-Server API specification and compare its account-data description with the common namespaced identifier grammar in the linked appendix. Update the specification to define the account-data type grammar and verify that the allowed characters, maximum length, and empty-value behavior are explicit.

Written by the indexing model from the issue text.