Consider consistency for the exercises

[kytrinyx]

• 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.