How to trigger language server to re-index

### Description
After adding packages to my `gradle.build.kts`, auto-imports for the new things weren't working for me. I was able to fix by restarting the language server from the command palette and waiting for the "kotlin: indexing..." status to finish.

I'm wondering if its possible to trigger a re-index without restarting the server?

Update: I notice from the extensions "Feature contributions", the only command exposed is `kotlin.languageServer.restart`

### Extension version
0.2.22

### Additional context
n/a

Start with the existing `kotlin.languageServer.restart` command shown in the extension's Feature contributions and trace the indexing status reported as `kotlin: indexing...`. Define a command that re-indexes after changes to `gradle.build.kts` without restarting the language server, and verify that auto-imports for newly added packages work afterward.

Written by the indexing model from the issue text.