Additional `List.intersperse` simplifications

[lue-bird]

List.intersperse a [] --> [] already exist. Possible additions:

List.intersperse a [ b ]
--> [ b ]
-- which includes for example
List.intersperse a << List.singleton
--> List.singleton

List.intersperse a [ b, c ]
--> [ b, a, c ]

I imagine the last one is controversial because you sometimes expect to add more elements to the list in the future or just want make the separators the rule, like

[ navigationPanelUi
, channelMessagesUi
]
    |> List.intersperse (Ui.spacer 10)
-->
[ navigationPanelUi
, Ui.spacer 10
, channelMessagesUi
]

Since we also don't for example fix List.sum [ a, b ] to a + b it probably makes sense to not add this check to simplify (but to something like miniBill/elm-review-optimize)

[jfmengels]

I'm in favor of the former one, but as you suggested not in favor for the last one (where the element gets introduced) for the reasons you mentioned.