Parses different filter formats.

Depending on the format and type of the filters argument, returns:

• a decoded map, when filters is encoded JSON
• a parsed filter map, when filters is a filter expression string
• the same map, when filters is a map

Returns an empty map when argument type is unexpected (e.g. nil).

Examples:

iex> Filters.parse("{\"page\":\"/blog/**\"}")
%{"event:page" => {:matches, "/blog/**"}}

iex> Filters.parse("visit:browser!=Chrome")
%{"visit:browser" => {:is_not, "Chrome"}}

iex> Filters.parse(nil)
%{}