Eliminate ad hoc creation of function completion items

[jennybc]

Loose end discovered while working on #819:

ark/crates/ark/src/lsp/completions/completion_item.rs

Lines 122 to 135 in bcdaed2

	if rhs.node_type() == NodeType::FunctionDefinition {
	if let Some(parameters) = rhs.child_by_field_name("parameters") {
	let parameters = context
	.document
	.contents
	.node_slice(&parameters)?
	.to_string();
	item.detail = Some(join!(label, parameters));
	}
	item.kind = Some(CompletionItemKind::FUNCTION);
	item.insert_text_format = Some(InsertTextFormat::SNIPPET);
	item.insert_text = Some(format!("{}($0)", label));
	}

completion_item_from_assignment(), which is used for document completions, hand crafts a function completion item, which means it doesn't benefit from all the smarts in completion_item_from_function().

I actually found this problem when writing tests for #819, because my first crack at it was with a user-defined function in the global environment. I wondered why things weren't working! This issue is a reminder to me to come back and route this through completion_item_from_function() and to look hard for any other instances of similar.