Adds dedicated import-only tasks backed by google-java-format's --fix-imports-only CLI mode.

New tasks:

• javafmtFixImports
• javafmtFixImportsCheck
• javafmtFixImportsAll
• javafmtFixImportsCheckAll

Why tasks instead of a setting?

--fix-imports-only is not just another formatting preference. It changes the meaning of the run from "format this file" to "only touch imports and leave the rest of the file alone". Because of that, modeling it as a persistent setting would be surprising:
enabling it would silently change what javafmt and javafmtCheck do.

Dedicated tasks make that behavior explicit:

• javafmt and javafmtCheck remain full formatting tasks
• javafmtFixImports and javafmtFixImportsCheck are clearly import-only operations

This also fits better with how people use it in practice. Import-only cleanup is usually something you invoke intentionally for a specific workflow, not a mode you want to keep enabled all the time.

Behavior details:

• javafmtFixImports... does not format the rest of the Java file
• it only runs the import-fixing step from google-java-format
• only javafmtRemoveUnusedImports and javafmtSortImports influence these tasks
• other formatting-related settings do not matter in this mode, because full formatting is skipped

This PR also adds scripted coverage for the new import-only tasks and documents them in the README.