Improve safety of RFormatter.importable (#353)

Previously, if the system was missing Rscript, an exception would be
thrown from RFormatter.importable. In some configurations, this
exception caused a noisy and annoying error message, "Unable to find
default formatters to use"

jupyterlab_code_formatter/formatters.py

@@ -361,6 +361,9 @@ def package_name(self) -> str:
 
     @property
     def importable(self) -> bool:
+        if not command_exist("Rscript"):
+            return False
+
         package_location = subprocess.run(
             ["Rscript", "-e", f"cat(system.file(package='{self.package_name}'))"],
             capture_output=True,