Provide JSDoc comments for flag properties

[tommy-mitchell]

Adds JSDoc comments to each property of the type Flag so that Intellisense is provided when creating a flag:

[tommy-mitchell]

I also added missing info to the flags JSDoc that was in the readme, and reordered the options to group similar concepts.

[tommy-mitchell]

I'd like to help get #211 merged before the next release goes out. Should I open a new PR branched from it, or should it be reopened and I make PRs to its feature branch?

[sindresorhus]

Should I open a new PR branched from it,

New PR.

[tommy-mitchell]

@sindresorhus unless you'd want to get #229 or #221 merged before the next release, you're good to put it out. Adding subcommands will probably be better with some general refactoring (separating utilities/steps into separate files, doing the same with tests/some fixtures, etc). I think that would be a good time to work on things like bundling and making errors consistent as well. I'll open a draft PR for that soon.

[sindresorhus]

There's no rush in getting out a new version. It's going to be a major bump.