Better C++/Clangd Support (default theme)

[lfod1997]

Adding some support for C++ development under Clangd (llvm-vs-code-extensions.vscode-clangd) in default theme.

• keyword.other.namespace.directive.cpp and keyword.other.namespace.definition.cpp → keyword (currently hitting storage.type)
• entity.other.attribute source.cpp → keyword (currently hitting entity.other.attribute, looks like a type in both themes)
• semantic variable: readonly functionScope → variable (currently hitting variable.other.constant)
• semantic class: deduced defaultLibrary globalScope ("auto" keyword in type deduction) → keyword (currently hitting semantic class)