[Feature] Make outline configurable for switch between PublishOutline and PublishFlutterOutline

[changyiyao]

Hi guys,

For now, the outline CocOutline only shows the class, variables, functions, methods which are dart relates.
I'm not quite familiar with dart LSP server, but I assume after adding support for PublishFlutterOutline notification the outline can show additional informations about flutter such as the widget hierarchy?

If the above is true, the feature description is:

1. Add a coc-config that makes LSP outline initialization option switch between PublishOutline and PublishFlutterOutline if they cannot be turns on at the same time.
2. CocOutline show the corresponding outline that we can do actions(search, select, actions just like CocAction) on each widget in flutter build methods.

Also request the outline for publishFlutterOutline notifications which may show flutter widget tree?

Please open a new issue for publishFlutterOutline

Originally posted by @iamcco in #108 (comment)