.gitignore: add zig-out, remove vim backup and swap files

Add the zig-out directory, since the install step is available.

Remove the vim backup and swap files, since it is possible to configure
vim to use a custom directory.

Ensure all the directories are anchored to the root directory.

Closes #251

.gitignore

@@ -1,5 +1,4 @@
-*~
+/zig-cache/
-*.swp
+/zig-out/
-zig-cache/
+/answers/
-answers/
+/patches/healed/
-patches/healed/