Fix `RemoveTrailingWhitespace` to preserve shebang lines

[timtebeek]

Summary

• Fix RemoveTrailingWhitespace recipe removing shebang lines from JavaScript files
• Add COMPILATION_UNIT_PREFIX to the list of locations to skip when removing trailing whitespace
• Add test case to verify shebang preservation in JavaScript files

Changes

• RemoveTrailingWhitespaceVisitor.java: Skip COMPILATION_UNIT_PREFIX location to preserve shebangs and other file-level prefixes
• RemoveTrailingWhitespaceTest.java (JavaScript): Add integration test for shebang preservation

Test plan

• [x] New JavaScript integration test passes: RemoveTrailingWhitespaceTest.doNotRemoveShebang()

• [x] All existing Java tests pass: RemoveTrailingWhitespaceTest

• [x] Verified the fix resolves the issue described in #6274

• Fixes #6274

🤖 Generated with Claude Code

[greg-at-moderne]

CI taking more than 1hr to run? Is it stuck or what's wrong with this?