add task 3

exercise2/part 1/.gitignore

exercise2/part 1/ChangeLog.md

exercise2/part 1/LICENSE

exercise2/part 1/README.md

exercise2/part 1/Setup.hs

exercise2/part 1/app/Main.hs

exercise2/part 1/app/Tokens.hs

exercise2/part 1/app/Tokens.x

exercise2/part 1/app/test.txt

exercise2/part 1/exercise2stack.cabal

exercise2/part 1/package.yaml

exercise2/part 1/src/Lib.hs

exercise2/part 1/stack.yaml

exercise2/part 1/stack.yaml.lock

exercise2/part 1/test/Spec.hs

exercise2/part 2/Main.hs

exercise2/part 2/Tokens.hs

exercise2/part 2/Tokens.x

exercise2/part 2/test.txt

exercise2/part 3/MDLTokens.x