Add a "fast mode" where dartdoc skips expensive computations

[jcollins-g]

@kwalrath It seems like it should be possible to build a preview mode for dartdoc where it doesn't generate links and/or doesn't generate complete docs for a package (only a given class/library). This may have restrictions on things like macros, but for simple cases might be able to make it faster to see the impact of editing documentation comments on the docs.

Something like this is definitely a good idea (P2) but might only get past prototyping if we can prove it is fast enough compared to regular doc generation to be worth it.

[kwalrath]

Another option is to use a non-dartdoc tool to allow people to check their doc code (with or without ///s). Maybe a dartdoc-ish version of https://dart-lang.github.io/markdown/.

[kwalrath]

/cc @srawlins