"Event" signal creator, emphasizing id + (optional level).

Default  kind: `:event`
Default level: `:info`
Returns:
  - For `event!`  variant: nil, unconditionally.
  - For `event!?` variant: true iff signal was created (allowed by filtering).

When filtering conditions are met [4], creates a Telemere signal [3] and
dispatches it to registered handlers for processing (e.g. writing to
console/file/queue/db, etc.).

Examples:

  (event! ::my-id)       ; %> {:kind :event, :level :info, :id ::my-id ...}
  (event! ::my-id :warn) ; %> {...           :level :warn ...}
  (event! ::my-id
    {:let  [x "x"] ; Available to `:data` and `:msg`
     :data {:x x}
     :msg  ["My msg:" x]}) ; %> {... :data {x "x"}, :msg_ "My msg: x" ...}

Tips:

  - Test using `with-signal`: (with-signal (event! ...)).
  - Supports the same options [2] as other signals [1].

  - `log!` and `event!` are both good default/general-purpose signal creators.
  - `log!` emphasizes messages, while `event!` emphasizes ids.

  - Has a different 2-arity arg order to all other signals!
    Mnemonic: the arg that's typically larger is *always* in the rightmost
    position, and for `event!` that's the `level-or-opts` arg.

----------------------------------------------------------------------
[1] See `help:signal-creators` - (`signal!`, `log!`, `event!`, ...)
[2] See `help:signal-options`  - {:keys [kind level id data ...]}
[3] See `help:signal-content`  - {:keys [kind level id data ...]}
[4] See `help:signal-filters`  - (by ns/kind/id/level, sampling, etc.)