Fix: defined default var for langchain cache path to enable custom path (issue 987)

[necjamahim]

When defining lc_cache_path and enabling cache in config:

# Enable third parties caching (e.g LangChain cache)
cache = true
lc_cache_path = './.custom-cache-path.db'

TypeError is thrown:

TypeError: chainlit.config.ProjectSettings() got multiple values for keyword argument 'lc_cache_path'