Implement global project directory locking

[lukaszcz]

The first thing the Juvix compiler should do is to lock the project directory to avoid unpredictable interference with other Juvix invocations. We tried fine-grained locking, but it was hard to get right because of Haskell's laziness. A global lock on the whole project directory acquired on startup should be easier to implement.

[paulcadman]

We don't want to lock the global project for vscode because it would make the extension too unresponsive. So we could add a flag to disable the global project lock.

The real solution to vscode responsiveness is lsp