Allow filename-modifiers for %e

In #2556, we implemented support for `filename-modifiers` on `%s` and `%t`, so you can work out paths relative to the filename being edited, or to a temporary file generated for checking a file. It would be nice to also be able to apply modifiers to `%e` for working out paths relative to the executable path. This may be useful for certain linters which need libraries including in an option, which are relative to the path of the executable.