"Event" signal call, focused on id + level.

API: [id] [id level-or-opts] => true iff signal call was allowed
Default  kind: `:event`
Default level: `:info`

When conditions are met [1], creates a Telemere signal [2] and dispatches it to
registered handlers for processing (writing to console/disk/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 as other signals [3].

  - A good general-purpose signal, prefer to `log!` by default, since it
    better encourages structured data over unstructured messages.

  - 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.

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[1] See `help:signal-handling` docstring
[2] See `help:signal-content`  docstring
[3] See `help:signal-options`  docstring