Add `atom-text-editor.autocomplete-plus-symbol-provider` to the set of textEditorSelectors for `SymbolProvider`.

[bolinfest]

If I create an <atom-text-editor> and want to get support from the SymbolProvider, I should not have to put it under the DOM structure atom-pane > .item-views > atom-text-editor in order to leverage it.

I think it would be a much better pattern for 3.0 providers to choose a marker interface based on their package name so that others can opt-in by doing:

<atom-text-editor class="autocomplete-plus-symbol-provider"></atom-text-editor>

[bolinfest]

/cc @as-cii