Elm-format: \xADf should not format to \x0ADF

[ahstro]

Inserting a soft hyphen using \xAD in front of the letter f will format format it to \x0ADF, e.g., "\xADfoobar" would turn into "\x0ADFoobar", effectively inserting a new unicode character and dropping the f from whatever word was used.

[avh4]

Duplicate of #412

[avh4]

Elm 0.19 will escape characters different, so the code will now be "\u{00AD}foobar", which will no longer be parsed incorrectly when formatted.

A workaround for Elm 0.18 and earlier is to change the code to "\xAD" + "foobar", which will format correctly.

[avh4]

This is fixed in master for Elm 0.19. This isn't going to be fixed for Elm 0.18 and earlier, as it's an invasive fix and isn't necessary in Elm 0.19 and beyond. (For Elm 0.18 and earlier, see the workaround mentioned here: https://github.com/avh4/elm-format/issues/462#issuecomment-390455947)