Add --attribute-help flag

[atomb]

Summary

It should be easy to get a list of what attributes Dafny supports and what each ones does.

Background and Motivation

The old-style options have an /attrHelp flag, and that text is available in Dafny already, so we should be able to access it with new-style options.

Proposed Feature

Add a --attribute-help flag to print help about all available attributes.

Alternatives

It's possible to use --boogie -attrHelp at the moment, but it's not obvious.

[keyboardDrummer]

The old-style options have an /attrHelp flag, and that text is available in Dafny already, so we should be able to access it with new-style options.

Because something is possible with the old CLI, does not mean it should be possible with the new one. The attributes are documented in the reference manual (here). Why would you want to be able to output a part of our reference manual using /attrHelp ? If you want that, then wouldn't it be more consistent to let the CLI to output the entire reference manual?