Servo has for years implemented a behavior where spacebar or enter click
on a focused element will trigger a click. That behavior used to be
unspecified, but now it is. The specification says that this should only
happen on state-changing elements. This change makes this happen using
activatable elements as a heuristic (as state-changing elements are not
defined in the specification). In addition, the behavior is integrated
into the default keyboard event handler code.
Testing: This change adds a Servo-specific test due to the ambiguities
in the specification.
Fixes: #39028.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>