LSP: Add option for modifying environment for language server process

On some platforms it is not easy to modify the environment for Qt Creator when started from menus or other indirect ways (macOS Finder/Spotlight/Dock, Ubuntu Dock/Favorites/Search), and some language server setups need changes in the system environment to work correctly.

Aside from that it might not be desired to change the environment for everything.

As an example, haskell-ide-engine on macOS requires $HOME/.local/bin to be on the PATH. Currently the (basically) only way to do this is to run Qt Creator from a Terminal with the environment correctly set up.