[READY] Add support for DocumentSymbol in document outline requests

[bstaletic]

I'm not too happy about this. Omnisharp LSP assumes clients support this. We could, but it comes at the cost of manually flattening the hierarchy. That's what this pull request does.

Alternatives include:

• Implementing this, but not advetising the capability. That way, servers that assume client support for it work, but those that don't won't have a performance impact.
• Leave this mess out of the LanguageServerCompleter class and only add this hack into the LSP Omnisharp completer. This was done in #1752

EDIT: Switched to the first alternative, as discussed in gitter.

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This change is 

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.92%. Comparing base (1026c83) to head (3027d22). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1757 +/- ## ========================================== + Coverage 95.90% 95.92% +0.01% ========================================== Files 84 84 Lines 8450 8462 +12 Branches 163 163 ========================================== + Hits 8104 8117 +13 + Misses 296 295 -1 Partials 50 50 ```

[bstaletic]

I'll merge this, to progress the C# stuff.