WIP: Add additional closing brace inlay hints

[markmurphydev]

Working on additional inlay hints for #18833, and looking for some guidance.

Currently we traverse the syntax tree, checking if the current node is a block-like expression, then figuring out the type of the parent.

• It seems possible to go parent node -> child block, which might be more readable. Any opinion one way or the other?

Currently I include almost every child of the parent syntax element (ex. function's visibility, params, types, ...) in the type hint, then join newlines and truncate the string to InlayHintsConfig.max_length.

• HirFormatter has a max_size field, but most of the implementations don't currently respect it. If I updated them to do so, and also added a configuration to exclude specific children, I think the text for each inlay hint could be gotten with a single call like parent.display_truncate_exclude(max_size: 40, exclude: vec![block_child]). Does that seem like a good way to go?
• I'm using format! on the AST nodes, because I had them already. If hir-format is not respecting max_size, is there still a reason to prefer HIR display functions?
• Closing hints look pretty bad when the original definition spanned multiple lines. I join the newlines, but line-beginning whitespace from child tokens (ex. function parameters) stays in. To me, the most obvious solution is to do a regex match for whitespace, but I think we don't want to pull in the regex crate?

• I added "// " before closing-brace inlay hints, to make clear that they don't represent code. Does it look ok?