Consider loose completions in the syntax server on unresolved/error types

[DanielRosenwasser]

Assume that no type named Foo exists in the current project, and assume the current file.

export let x: Foo;

x./**/

When requesting completions at /**/, we should consider giving a list of identifiers in the current file, similar to what we do for any in JavaScript files. This contrived example isn't great, but when opening a larger file, this could be helpful.

This is somewhat related to

• Heuristic/loose completions for the 'any' type #5334, where we considered giving identifier completions whenever hitting any
• Provide a better display experience for unresolved values and types #39032, where we discussed producing these special loading types and unresolved error types.