5687: Fix document symbols order r=matklad a=magurotuna
Resolves#5655
And adds tests for `handle_document_symbol`, both with `hierarchical_symbols` enabled and with it disabled.
Previously document symbols were displayed in reverse order in sublime text with its LSP plugin, but this patch fixes it like this:
![image](https://user-images.githubusercontent.com/23649474/89709020-fbccce00-d9b6-11ea-83b0-c88dc9f7977f.png)
Co-authored-by: Yusuke Tanaka <yusuktan@maguro.dev>