Autocompletion should list arguments beginning with `_` last

[omentic]

Description

See PgBiel/elembic#85. This is a particular problem with Elembic, but it is an issue with any package that uses the convention of __ prefixes for internal argument junk.

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[Alex-Muirhead]

This would definitely be a useful feature.
When working with writers who have less typst knowledge (i.e. they want it to "just work"), they can be confused by these "strange" reference suggestions popping up.