Add hotkey to open cppreference documentation

[sr-tream]

This PR add hotkey Shift+f1 to open documentation on cppreference.com, like in CLion.

I added this feature in clangd extension, because VSCode do not provide API to get symbol info from LSP, and standalone doc extensions can't get symbol scope.

Example:

std::string foo;
auto bar = foo.da^ta();

For this code, standalone extensions opens doc for std::data. With this PR clangd open doc for std::string::data