Negation in filters

I feel like the `foobar` and `no_foobar` idiom used in the filters is quite inelegant. As there is already a special character involved in the patterns (`*`) I propose to introduce a second one `^` which could be prepended to a string to negate it. I was first thinking about the `!` character but it would clash with room ids, `^` seems the second logical one to me because of it's use in regex character-class negation (we could also consider `-` or `~`).

reference: [filter spec](https://matrix.org/docs/spec/client_server/r0.2.0.html#post-matrix-client-r0-user-userid-filter)

Start with the linked Matrix filter specification and review how filter patterns currently define the foobar and no_foobar idiom. Clarify the negation character and its semantics, then update the specification so the supported syntax and examples are unambiguous.

Written by the indexing model from the issue text.