Refactor and use already implemented format functions for unblocked todos for `sway-fmt-v2`

[kayagokalp]

There are some todos in various places in sway-fmt-v2 to be implemented later (because they were blocked at that time) like

sway/sway-fmt-v2/src/items/item_abi.rs

Line 54 in 22859f3

item.0.value.span().as_str(), // TODO(PR #2173): FnSignature formatting

. Since we have most of the formatter implemented we can fix those. This issue is for tracking this kind of old todos.