Improve semantics of library organization

Currently, the directory Library/ is treated as a hard-coded special case by the lean project-create and lean library add commands. Given that projects list the names and paths of their library dependencies and that libraries also have config.json files, it seems that dropping the hard-coded value in favor of more explicit settings/flags would be cleaner and allow users more flexibility for code organization.

This assumes that the runtime doesn't treat Library/paths as special, which I have not checked.

Current status:
is_lean_library() checks whether the argument to library add starts with "Library/" to determine its return value, and this is what the library add command uses to determine whether to add a local library or to install from pypi/nuget.

The docs say to use project-create to create a new library. [project_create] looks at whether the path to the project contains "Library/" and if so, uses the library template and puts it in Library.py (instead of main.py with the usual algo contents)
https://github.com/QuantConnect/lean-cli/blob/master/lean/commands/create_project.py#L428-L434
https://github.com/QuantConnect/lean-cli/blob/master/lean/commands/create_project.py#L455-L457

Proposed changes:

• introduce an argument to project-create, --library to indicate that the project is intended to be a library
• when "Library/" is in the path according to current logic, but --library is not set to true, emit warning log message
• is_library_project = is-library-arg OR current logic
• modify project-create so that it also sets is-library in config.json when creating a library
• modify is_lean_library() in to use the field in the config rather than the path

Optional / nice-to-have changes:

• introduce library-create command, analogous to project-create, but for creating libraries
• factor out common logic between the two commands
• add a deprecation warning log message to project-create in the library-creation path
• introduce an argument to the library add to explicitly indicate whether the library is local or to be installed from pypi/nuget

Start with lean/components/util/library_manager.py, especially is_lean_library(), and the project-create and library add entry points in lean/commands/create_project.py and lean/commands/library/add.py. Trace how project paths and config.json are handled before deciding how the explicit library setting should interact with the existing Library/ logic. Done means library projects can be identified from configuration and project-create supports the proposed flag without relying solely on the hard-coded path.