feat(grapple-nvim): better keymaps + add nvim-web-devicons dependency

[cbochs]

📑 Description

Hello, again 👋

I received a bit of feedback after releasing the rewrite the other day. Thought I would send one more PR to address that here as well 🙂

Changes

• Add nvim-tree/nvim-web-devicons as a lazy (optional) dependency
• Use toggle_* instead of open_* so Grapple windows can be open/closed with the same mapping

[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.

[Uzaaft]

@cbochs Thank you for the PR!