Support hierarchical document symbols

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Needs support from both LSP client and server. Falls back to grouping symbols by kind when not supported. Uses CoC data model and renderer.

[liuchengxu]

Is there any language server that I can try this without using coc?

[greye]

Is there any language server that I can try this without using coc?

I'm using modified vim-lsc (had to set proper capability) with clangd. Basically, any compatible client-server pair should work the same provided both have support and capabilities set.