Allow for [Completion|Symbol]Kind modifiers

[jrieken]

extracted from #2628

In completions and for symbols we allow to provide a kind which is mostly use to derive an icon. For instance vscode.CompletionItemKind.Method. This issue is about allowing to modify these kinds to expressing that something is a "private member" or an "abstract protected method"

The idea is that we don't add more and more kinds, e.g no CompletionItemKind.PrivateMethod but a second type, a kind-modifier, that works in combination with a kind.

Potential candidates are: public, protected, internal, package-private, friend, private, static, final, const, abstract, volatile, transient, ... A "n languages have this"-rule should be applied when picking candidates.

[jrieken]

cc @kieferrm

[jrieken]

More modifier candidates are: deprecated, experimental

[sean-mcmanus]

For C/C++, we would like "declaration", "private", and "protected".