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

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 https://github.com/iamcco/coc-flutter/issues/108#issuecomment-917361110_