KiCad: add *.kicad_sch-bak and *.kicad_prl (#3427)

* KiCad: add *.kicad_sch-bak

As used by the new file formats for KiCad 6.0
See https://kicad-pcb.org/blog/2020/05/Development-Highlight-New-schematic-and-symbol-library-file-formats-are-now-the-default/

* Add *.kicad_prl to KiCad.gitignore

More info: https://forum.kicad.info/t/new-project-file-format/23705