BSP: Detect and recover from changed mill versions

[lefou]

The current BSP server implementation depends on the exact Mill version. Each version change may result in a changed classpath, hence we currently require to re-run the install command mill mill.bsp.BSP/install. Unfortunately, this is something easily to forget, so we should better handle it, even if that means, the first start after a version change is a bit slower.

Background:

When installing, we generate a resource file which contains the extended classpath containing the BSP server implementation (worker).

After a version change, this resource file is not present and Mill fails to start the server.

Solution:

Start in some light mode, resolve and reinstall the resource file and restart the server.

If necessary, start with old server version first and speak with BSP client.

[lefou]

The BSP spec allows a BSP server to update the BSP connection file at any time, so it is probably ok and a good idea to re-generate it and then re-start the server.