Add templates for exercises batch 6

[meatball133]

Includes:

• Largest series product
• Leap
• Linked list
• Luhn
• Matching brackets
• Matrix

Note:

Linked list have breaking changes in form of adding two extra methods that exist in problem spec.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[kotp]

Approved, with normal historical nag, and suggested replacement phrasing.

[kotp]

Suggested change

	raise "Implement this yourself instead of using Ruby's implementation."
	raise "Write this functionality yourself instead of using Ruby's `leap?` method."

[kotp]

Because "implement the implement you implement yourself, rather than the implement that Ruby implements" can be very confusing, if accurate and proper use, but I tend to avoid this word in multicultural and multilingual audiences.

English is hard enough as it is, using a word with 4 senses when people tend to not take care or be clear which sense they are talking about can be doubly hard for those that do not have English as a first language.