Add REUSE-compliant copyright headers

.gitignore

COPYING

ChangeLog

LICENSES/MIT.txt

Makefile

README.md

build.zig

ncdu.pod

src/browser.zig

src/delete.zig

src/main.zig

src/model.zig

src/ncurses_refs.c

src/scan.zig

src/ui.zig

src/util.zig