Add support for a insert_final_newline setting

[aslilac]

...which would do basically what it sounds like. If the file does not end with a newline, then add one before actually writing to the disk when invoking :w.

There's lot of precedent for a setting like this such as "files.insertFinalNewline in VSCode or the more general insert_final_newline setting specified by EditorConfig.

I'm very used to having VSCode do this for me, and I very much miss it when using Helix. I very often forget to add it when making a new file, only to sob quietly when I see the red little "you dun goofed" icon on Github when looking at it later.

[zqianem]

Done now that #8157 is merged 🎉

[aslilac]

oh sweet!