Add missing practice exercises

Hey,

This is a list of all missing (not deprecated) exercises in the C++ track.

Let me know which one you’re picking up, and then go ahead and start the PR.
I’ll maintain and update the list accordingly.

- [ ] baffling-birthdays (@marcelweikum) #977
- [ ] book-store (@marcelweikum) #974
- [ ] bottle-song (@marcelweikum) #979
- [ ] bowling (@marcelweikum) #982
- [ ] change (@marcelweikum) #983
- [ ] connect (@marcelweikum) #984
- [ ] custom-set
- [ ] dominoes
- [ ] dot-dsl — *(canonical-data.json missing)*
- [ ] error-handling — *(canonical-data.json missing)*
- [ ] flatten-array
- [ ] forth
- [ ] game-of-life
- [ ] go-counting
- [ ] grep
- [ ] hangman — *(canonical-data.json missing)*
- [ ] house
- [ ] intergalactic-transmission
- [ ] killer-sudoku-helper
- [ ] ledger
- [ ] lens-person — *(canonical-data.json missing)*
- [ ] markdown
- [ ] matrix
- [ ] micro-blog
- [ ] ocr-numbers
- [ ] paasio — *(canonical-data.json missing)*
- [ ] palindrome-products
- [ ] piecing-it-together
- [ ] poker
- [ ] pov
- [ ] proverb
- [ ] pythagorean-triplet
- [ ] rational-numbers
- [ ] react
- [ ] rectangles
- [ ] relative-distance
- [ ] resistor-color-trio
- [ ] rest-api
- [ ] saddle-points
- [ ] satellite
- [ ] sgf-parsing
- [ ] simple-cipher
- [ ] split-second-stopwatch
- [ ] square-root
- [ ] state-of-tic-tac-toe
- [ ] strain
- [ ] swift-scheduling
- [ ] tournament
- [ ] transpose
- [ ] tree-building — *(canonical-data.json missing)*
- [x] twelve-days (@alienx5499) #992
- [ ] variable-length-quantity
- [ ] word-search
- [ ] wordy
- [ ] zipper

Not sure how to get started? This might help: [Practice Exercises](https://exercism.org/docs/building/tracks/practice-exercises)

Happy coding!