Parses different filter formats.

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

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

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

Examples:

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

iex> Filters.parse("visit:browser!=Chrome")
[[:is_not, "visit:browser", ["Chrome"]]]

iex> Filters.parse(nil)
[]