Add categories to pubspec.yaml

[mnordine]

Search results on https://pub.dartlang.org/ could be improved with the addition of categories for packages. It could also improve the home page by listing packages via popular categories. These categories must match a pre-defined list. For prior art, see https://crates.io/ and http://doc.crates.io/manifest.html

From the previous link:

# This is a list of up to five categories where this crate would fit.
# Categories are a fixed list available at crates.io/category_slugs, and
# they must match exactly.
categories = ["...", "..."]

[mnordine]

Related: #30418

[anders-sandholm]

This issue was moved to dart-lang/pub#1685