[ty] Pick up typed text as a query for unimported completions
It's almost certainly bad juju to show literally every single possible
symbol when completions are requested but there is nothing typed yet.
Moreover, since there are so many symbols, it is likely beneficial to
try and winnow them down before sending them to the client.
This change tries to extract text that has been typed and then uses
that as a query to listing all available symbols.
