feat(keyboard): hold modifiers for keyboard and mouse input inside a block

[route]

Fixes #581

Chrome doesn't carry a pressed modifier over to later input events: each CDP event needs its own modifiers, so a shift+drag required raw Input.dispatchMouseEvent calls.

page.keyboard.hold(:shift) do
  page.mouse.move(x: 10, y: 10).down.move(x: 200, y: 200, steps: 10).up
end

• Keyboard#hold(*modifiers) { } presses the modifiers, makes every keyboard and mouse event in the block carry them and releases them when the block returns or raises. Nested blocks press a key once, and a release failure after the block raised doesn't hide the original error. State lives only for the block, so a missing keyup can't leak into later input.
• Typing respects held keys: hold(:shift) { type("abc") } types ABC, holding Cmd/Ctrl and typing a selects all.
• Mouse#move accepts modifiers: like down and up. An explicit value overrides the held keys for that event.
• Mouse#click(modifiers:) now sends the modifiers with its mousemove too.
• Keyboard#down/#up raise ArgumentError for a modifier and point to hold. down(:shift) used to crash with NoMethodError and up(:shift) sent an empty keyup event.