Support special attributes :let, :for, :stream

[connorlay]

Adds supports for HEEx special attributes: :let, :for, and :stream.

A new node has been added to the grammar, special_attribute, that behaves similarly to the attribute node with the following changes:

• a special_attribute must have a corresponding expression node.
• a special_attribute can only be added to the following tags: start_component, start_slot, start_tag, and self_closing_tag.
• a special_attribute_name can only be one of the following values (more could be added in the future): let, for, stream.
• a special_attribute_name is highlighted as keyword

This PR also adds the prettier code formatter with configuration borrowed from tree-sitter-elixir.

[the-mikedavis]

Should there be a distinct highlight for special_attribute_name? @keyword might be a good fit

[connorlay]

Should there be a distinct highlight for special_attribute_name? @Keyword might be a good fit

Ah good call, I think there should be!

[connorlay]

@the-mikedavis Once this is merged I'll open a PR against Helix and Neovim to update the highlights 🙂

[the-mikedavis]

Looks great! 🚀