This PR significantly improves the test coverage of the language server,
providing at least a single basic test for every request that is used by
the client. It also implements infrastructure for testing all of these
requests, e.g. the ability to run interactive tests in a project context
and refactors the interactive test runner to be more maintainable.
Finally, it also fixes a small bug with the recently implemented unknown
identifier code actions for auto-implicits (#10442) that was discovered
in testing, where the "import all unambiguous unknown identifiers" code
action didn't work correctly on auto-implicit identifiers.
This PR ensures that deprecated declarations are displayed with a
strikethrough markup in the completion popup of VS Code and that the
docstring of a completion item denotes the meta-data of the deprecation.
