fix(mouse): preserve held button during movement

[sean0407]

Summary

Mouse#move
  keep the existing buttons bitmask
+ send the pressed button, or "none" while idle
  -> Chrome can recognize drags and emit Input.dragIntercepted

Fixes #635.

Evidence

• Before: bundle exec rspec spec/mouse_spec.rb:60 failed because Input.dragIntercepted was not emitted.
• After: bundle exec rspec spec/mouse_spec.rb — 18 examples, 0 failures, 10 pending.
• After: bundle exec rubocop lib/ferrum/mouse.rb spec/mouse_spec.rb — 2 files inspected, no offenses.
• Patch policy: 3 text files, 27 changed lines, no binary changes.

Merge Danger

Door: Two-way

Blast Radius: Narrow