Add Document Symbol Provider sample

[dhonx]

This is a forgotten one but is very necessary to help extension developers.

[franck44]

Thanks @dhonx @jrieken
I can confirm that it would be very nice to have an example.
I have tried to use the DocumentSymbol type but to no avail.
Using the old SymbolInformation worked well but of course lacks hierarchy.

There are a few resources on the topic including this one but it did not work for me (and I did not manage to debug it even by logging extra info in the console).

[coolcorexix]

I need this sample very badly too. the vscode.executeDocumentSymbolProvider currently do not work as I expected :(

the error I am having: microsoft/vscode#156738

[deanmaster]

hi all, is there any chance we have this example please :( I need this for hightlighting our own keyword. thanks a lot