Adding dialect labels

I was thinking that it could be nice to add dialect-based labels to the PRs, such as PostgreSQL, SQLite etc, so that reviewers familiar with each of these dialects can appropriately filter PRs. Maybe we can even add an auto-labeller bot.

@alamb @iffyio if you agree I will proceed to do so.

No files, tests, or entry points are named. First clarify whether the scope is dialect labels only or also an auto-labeller bot, and agree on the dialect list and labeling rules; done would mean the agreed labels exist and any agreed automation applies them consistently.

Written by the indexing model from the issue text.