Merge #6706

6706: Move import text edit calculation into a completion resolve request r=matklad a=SomeoneToIgnore

Part of https://github.com/rust-analyzer/rust-analyzer/issues/6612 (presumably fixing it)
Part of https://github.com/rust-analyzer/rust-analyzer/issues/6366 (does not cover all possible resolve capabilities we can do)
Closes https://github.com/rust-analyzer/rust-analyzer/issues/6594

Further improves imports on completion performance by deferring the computations for import inserts.

To use the new mode, you have to have the experimental completions enabled and use the LSP 3.16-compliant client that reports `additionalTextEdits` in its `CompletionItemCapabilityResolveSupport` field in the client capabilities.
rust-analyzer VSCode extension does this already hence picks up the changes completely.

Performance implications are descrbed in: https://github.com/rust-analyzer/rust-analyzer/issues/6633#issuecomment-737295182

Co-authored-by: Kirill Bulatov <mail4score@gmail.com>