[ty] Add builtins to completions derived from scope (#18982)

Most of the work here was doing some light refactoring to facilitate
sensible testing. That is, we don't want to list every builtin included
in most tests, so we add some structure to the completion type returned.
Tests can now filter based on whether a completion is a builtin or not.

Otherwise, builtins are found using the existing infrastructure for
`object.attr` completions (where we hard-code the module name
`builtins`).

I did consider changing the sort order based on whether a completion
suggestion was a builtin or not. In particular, it seemed like it might
be a good idea to sort builtins after other scope based completions,
but before the dunder and sunder attributes. Namely, it seems likely
that there is an inverse correlation between the size of a scope and
the likelihood of an item in that scope being used at any given point.
So it *might* be a good idea to prioritize the likelier candidates in
the completions returned.

Additionally, the number of items introduced by adding builtins is quite
large. So I wondered whether mixing them in with everything else would
become too noisy.

However, it's not totally clear to me that this is the right thing to
do. Right now, I feel like there is a very obvious lexicographic
ordering that makes "finding" the right suggestion to activate
potentially easier than if the ranking mechanism is less clear.
(Technically, the dunder and sunder attributes are not sorted
lexicographically, but I'd put forward that most folks don't have an
intuitive understanding of where `_` ranks lexicographically with
respect to "regular" letters. Moreover, since dunder and sunder
attributes are all grouped together, I think the ordering here ends up
being very obvious after even a quick glance.)

crates/ty_server/src/server/api/requests/completion.rs

@@ -56,7 +56,7 @@ impl BackgroundDocumentRequestHandler for CompletionRequestHandler {
             .into_iter()
             .enumerate()
             .map(|(i, comp)| CompletionItem {
-                label: comp.label,
+                label: comp.name.into(),
                 sort_text: Some(format!("{i:-max_index_len$}")),
                 ..Default::default()
             })