internal: Migrate convert_bool_then to SyntaxEditor

[ShoyuVanilla]

Part of #18285

[ShoyuVanilla]

Oh, I accidentally added part of another migration - convert_closure_to_fn - I was working on 😅
I'll continue working on it in a follow-up PR or this PR, if this isn't merged by then

[DropDemBits]

I've only looked at the convert_bool_then-related changes, but looks good to me & approving those changes.

Waiting on convert_closure_to_fn changes to be split out. Surprised that it's using splice_children in the first place, but shouldn't be too hard to replace it with the SyntaxEditor equivalent.

[ShoyuVanilla]

Dropped unfinished convert_closure_to_fn and will address it in a new PR

[Veykril]

This does still use AstNodeEdit for indentation, I assume that's mainly to not regress indentation for now? (merging either way)

[ShoyuVanilla]

Well, currently we have two ways to indent things (except manually indenting all lines in a syntax node):

rust-analyzer/crates/syntax/src/ast/edit.rs

Lines 119 to 133 in 81ff38f

	/// Soft-deprecated in favor of mutable tree editing API `edit_in_place::Ident`.
	pub trait AstNodeEdit: AstNode + Clone + Sized {
	fn indent_level(&self) -> IndentLevel {
	IndentLevel::from_node(self.syntax())
	}
	#[must_use]
	fn indent(&self, level: IndentLevel) -> Self {
	fn indent_inner(node: &SyntaxNode, level: IndentLevel) -> SyntaxNode {
	let res = node.clone_subtree().clone_for_update();
	level.increase_indent(&res);
	res.clone_subtree()
	}
	Self::cast(indent_inner(self.syntax(), level)).unwrap()
	}

rust-analyzer/crates/syntax/src/ast/edit_in_place.rs

Lines 964 to 970 in 81ff38f

	pub trait Indent: AstNode + Clone + Sized {
	fn indent_level(&self) -> IndentLevel {
	IndentLevel::from_node(self.syntax())
	}
	fn indent(&self, by: IndentLevel) {
	by.increase_indent(self.syntax());
	}

and both of them utilizes mutable syntax trees.l

I'll add a non-mutable syntax tree alternative and replace previous usages with it in next PR 😄