Find the sclang executable.

The following paths, if defined, will be searched (prioritised as ordered): 1. The absolute path sclang_path 2. The environment variable SCLANG_PATH 3. sclang_path if defined in Supriya’s configuration file 4. The user’s PATH 5. Common installation directories of the SuperCollider application.

Returns a path to the sclang executable. Raises RuntimeError if no path is found.