Gitignore stack.yaml.lock

Problem: with `stack-2+`, `stack.yaml.lock` file appears on every build.

Solution: pleasing developer's lazyness and with a light hand ignoring
the best practices, we add the lock file to gitignore.

.gitignore

@@ -16,6 +16,7 @@ cabal.sandbox.config
 *.hp
 *.eventlog
 .stack-work/
+stack.yaml.lock
 cabal.project.local
 cabal.project.local~
 .HTF/