[FEATURE] Project Symbols Request

[SpontanCombust]

Implement search like in #53, but make it work for dependencies, most importantly content0.

[SpontanCombust]

Decided to use the regular "workspace symbols" request to also show symbols in dependencies. A setting (by default turned on) is used to judge whether the search should be extended beyond the workspace:
languageServer.extendedSearchForWorkspaceSymbols