feat(install): add --keepsrc to keep pkg/ and src/ directories (#2272)

* feat(install): add --nocleanbuild to keep pkg/ and src/ directories for
AUR packages

Providing this flag during installation of AUR packages allows for keeping
the src/ and pkg/ directories produced my makepkg. If the user wants to
delete the directories, they can either select to cleanBuild in the
cleanmenu or run the installation without the --nocleanbuild flag (yay
will only remove the directories if the package is rebuilt)

* fix(completion): simplify description for --nocleanbuild in fish

This makes the description consistent with the descriptions in the
man page, --help, and zsh completion.

* refactor(install): Rename --nocleanbuild to --keepsrc

This naming scheme is more familiar to users since it is the name of the
flag in Paru.

---------

Co-authored-by: jguer <me@jguer.space>

pkg/settings/parser/parser.go

@@ -378,6 +378,7 @@ func isArg(arg string) bool {
 	case "save":
 	case "afterclean", "cleanafter":
 	case "noafterclean", "nocleanafter":
+	case "keepsrc":
 	case "devel":
 	case "nodevel":
 	case "timeupdate":