UI - Support expandable blocks

[ericdallo]

We should support expandable blocks for reason and tool calls, like other editors, this is critical for ECA chat UX

Considerations:

• User should be able to toggle the expandable block with keybindings (tab, enter)
• eca-nvim should be able to update a expandable block content via a ID (reaasonId, toolCallId)

[joaopluigi]

done at #53