Add visibility-related (e.g. public/protected/private) symbol tags #2140

For a language like Erlang, it would be convenient to distinguish exported/non exported (think of public/private) functions in the file outline or when jumping to a symbol.

The outline is powered by the "documentSymbol" request, where each symbol has a LSP symbol Kind associated.

Symbol tags can also be associated to symbols, but the only tag available right now is the "deprecated" one.

I suggest we add visibility-related tags to indicate the visibility of a symbol. This would be consistent with the already present "deprecated" symbol tag and enable the UI to present private/public symbols in slightly different ways (e.g. with slightly greyed out text, similar to what can already happen via semantic highlighting).