Autoimport on completion

Then you autocomplete function from another unit it's very comfortable to not import unit by another action
E.g
```
-- Before
import Data;

fold

-- After
import Data;
import List;

foldl()
```

See `org.jetbrains.lama.editor.completion.IdentifierCompletionProvider`,
`org.jetbrains.lama.editor.completion.LamaLookupElementFactory`

Start with org.jetbrains.lama.editor.completion.IdentifierCompletionProvider and org.jetbrains.lama.editor.completion.LamaLookupElementFactory to trace how completion items are created. Determine how selecting a function from another unit can update imports, then verify that completing foldl adds import List while preserving existing imports and caret placement.

Written by the indexing model from the issue text.