Fix merging auto-imports with existing import statements

[dfreeman]

I wasn't able to come up with a way to get tsserver to produce merged imports itself, but this should fix #1.

An unfortunate side effect of the intermediary transformed modules we use in @glint/tsserver-plugin means that from TypeScript's perspective, there's never an existing import for the non-transformed module to merge with, and the logic that generates import completions is completely private within the TypeScript codebase, so we can't reuse it.

Having to maintain this code ourselves would be one point in favor of using a separate file extension + standalone language server for files with embedded templates, but it's not clear that outweighs all the other advantages we get from the current approach. The whole "tsserver plugin vs standalone language server" issue is likely worth a writeup on its own to capture the tradeoffs inherent in each approach.