Use triple-slash comments for documentation

I propose that the Wit format use /// comments, rather than //.

Wit itself no longer has non-documentation comments, however multiple projects are looking at using Wit as a base language that they can extend with more features. For such languages, it will be desirable to be able to have both documentation and regular comments, while remaining a superset of Wit.

Using /// for documentation and // for regular comments is a common convention, found in rustdoc, doxygen, C#, Swift, and others.

In the short term, the wit tooling can continue to accept both /// and // as documentation comments, until existing Wit files are migrated. In the long term, wit tooling could reject // comments to free up their space in the grammar for derived languages.

@peterhuene @lukewagner @esoterra

Start by reviewing the proposal and its linked component-model issue 273, then locate the Wit grammar and comment-handling entry points in the repository. Determine whether both /// and // should remain accepted during migration and what the long-term grammar should reject. Done requires an agreed syntax policy and corresponding tooling changes, but no files or tests are named in the issue.