Add magic command to create aliases for AI models

[JasonWeill]

Problem

Currently, some AI models have aliases. For example, gpt2 is an alias for huggingface_hub:gpt2. These are defined in a constant in our Python code:

jupyter-ai/packages/jupyter-ai/jupyter_ai/magics.py

Lines 15 to 20 in b06526b

	MODEL_ID_ALIASES = {
	"gpt2": "huggingface_hub:gpt2",
	"gpt3": "openai:text-davinci-003",
	"chatgpt": "openai-chat:gpt-3.5-turbo",
	"gpt4": "openai-chat:gpt-4",
	}

To add a new alias, one would have to modify the file above and make a new release of Jupyter AI for everyone. There are no aliases for local usage.

Proposed Solution

Add a command %%ai alias <name> <value> to create custom aliases. These aliases persist for as long as the kernel remains running.

Add a command such as %%ai alias list that shows users which aliases they have defined. %%ai list (#28) may also display these aliases.

Update the example notebooks and documentation with information about this new command.