Isolate and more broadly utilize "expression split".

[amcgregor]

Currently only one directive attempts to split anything on an expression, the cinje.inline.text processor when handling %{expr arg} formatted string replacements. This is broadly useful, notably for the : use and : using directives amongst others, and better aligns with the documentation.

• [ ] Extract the cinje.inline.text "expression split" code into the Line class. (As a companion for the partitioned attribute.)
• [ ] Update cinje.inline.use directive to utilize the expression split.
• [ ] Update cinje.block.using directive to utilize the expression split.