Sync `triangle` and add stub

[BNAndras]

This syncs the docs, metadata, and tests for triangle. This adds two tests for scalene triangles that check other combinations of the two sides being equal. I also added an non-compiling stub that gives the student the minimum interface needed to pass the tests.

After this, there are 37 exercises where configlet is reporting one or more tests missing since the test.toml files were last updated.