Consider consistency for the exercises

- Is there a style guide for Ballerina?
- Are these styles encouraged or enforced?
- Are there any conventions that we should adopt on this track for the sake of consistency?
- Can we enforce these?
- Is there a linter? Are there many? Should we use one?
- When you add a linter, edit the pull request template [link]
- Update the pull request template with checks that are appropriate for this track
- Is there a common convention for filenames? If not, what should our convention be?

Note that this is about the exercises (the test suites and code examples), not people's solutions.