fix: correct highlight interpolation in explain message

[ckipp01]

Currently the output of explain when you are missing a return type in
an abstract declaration looks like this:

scala> trait Foo:
     |   def foo
-- [E019] Syntax Error: --------------------------------------------------------
2 |  def foo
  |         ^
  |         Missing return type
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | An abstract declaration must have a return type. For example:
  |
  | trait Shape {hl(
  |   def area: Double // abstract declaration returning a Double
  | )}
   -----------------------------------------------------------------------------

This fixes the interpolation issue so the return correctly shows:

scala> trait Foo:
     |   def foo
-- [E019] Syntax Error: --------------------------------------------------------
2 |  def foo
  |         ^
  |         Missing return type
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | An abstract declaration must have a return type. For example:
  |
  | trait Shape:
  |   def area: Double // abstract declaration returning a Double
   -----------------------------------------------------------------------------

The def area: Double is now also correctly colored.

[michelou]

@ckipp01 👍