add functionality to show the type hierarchy

[devoncarew]

In other tools, this is hooked up to something like F4. Those tools are able to show a tree view of the type hierarchy (superclasses and subclasses). We may have to expose this info in a vscode selection list (https://code.visualstudio.com/docs/extensionAPI/vscode-api#_window), so would need to flatten it somehow.

[DanTup]

Seems like a good idea; presumably will look similar to the document symbol list.

If it's just inheritance, won't this already be a flat list? Or are mixins/interfaces included too?

[devoncarew]

It could be a flat list going up, if you omit interfaces and such. Going down it'd be more difficult - easy to show children, harder to organize the grandchildren -

[DanTup]

Ah, of course; didn't think about children!

I presume this is the sort of thing you mean?

[devoncarew]

Yup; we'll need to flatten it somehow. That may be some creative naming of
items, or choosing to omit some levels of the tree.

[DanTup]

Had a quick look at this... Seems we can't use spaces to indent, not have multiple spaces together, they get stripped (including literal non-breaking spaces). Any character I use to try and force the indent looks lame:

Ideally, we'd have real indenting and be able to add icons and such, so it looks similar to the document symbol list:

Not sure how well we'll be able to do this. I've added an additional comment to #133 asking if they do it, could they consider use for this as well as just a view of symbols in the current document.

[DanTup]

I pushed the branch that has a command wired up to F4 and renders these dummy values to safe some time if you wish to have a play around:

https://github.com/Dart-Code/Dart-Code/tree/type-hierarchy