Fix indentation with interpolated strings

  * Add `on_embexpr_end()`.
  * Remove `alias_method "on_embexpr_end", :on_backref`
Add a test for an interpolated string
Remove an unnecessary argument
Use `do`, `end` indentation for string interpolation
Add a test for a nested interpolated string
Add embexpr_beg and embexpr_end to FONT_LOCK_NAMES