Indent current token as a "separator".
By "separator", we mean here a token whose sole purpose is to separate
various elements within some enclosing syntactic construct, and which does
not have any semantic significance in itself (i.e. it would typically no exist
as a node in an abstract syntax tree).
Such a token is expected to have an associative syntax and be closely tied
to its syntactic parent. Typical examples are "," in lists of arguments
(enclosed inside parentheses), or ";" in sequences of instructions (enclosed
in a {..} or begin..end block).
METHOD should be the method name that was passed to `smie-rules-function'.
Only meaningful when called from within `smie-rules-function'.
(fn METHOD)