feat(pack): add Lean language pack

[rami3l]

📑 Description

This PR adds language support for the Lean theorem prover via the lean.nvim extension.

This is an intentionally trivial PR to get my feet wet with config upstreaming :)

ℹ Additional Information

The lean.nvim plugin explicitly requires an on_attach() callback to be set, so I relied on the astrolsp-provided one.

[github-actions]

Review Checklist

Does this PR follow the [Contribution Guidelines](development guidelines)? Following is a partial checklist:

Proper conventional commit scoping:

• If you are adding a new plugin, the scope would be the name of the category it is being added into. ex. feat(utility): added noice.nvim plugin

• If you are modifying a pre-existing plugin or pack, the scope would be the name of the plugin folder. ex. fix(noice-nvim): fix LSP handler error

• Pull request title has the appropriate conventional commit type and scope where the scope is the name of the pre-existing directory in the project as described above

• README is properly formatted and uses fenced in links with <url> unless they are inside a [title](url)

• Proper usage of opts table rather than setting things up with the config function.

[mehalter]

LGTM! Thanks!