TreeView should use new outline feature

[ntotten]

Description Instead of the custom treeview, you could now use the built in outline feature of VS Code. By implementing it through the language server you would also enable the new breadcrumbs feature: https://code.visualstudio.com/updates/v1_27#_breadcrumbs-improvements

Right now XML doesnt support breadcrumbs:

[angelozerr]

Please try redhat.vscode-xml which provides this features:

[stale[bot]]

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