Jumping on sloping ground is broken

[Malcolmnixon]

PR #519 broke jumping while running on a sloped ground. Specifically the following test is broken:

godot-xr-tools/addons/godot-xr-tools/player/player_body.gd

Lines 350 to 353 in c5c7433

	# Skip if our vertical velocity is not essentially the same as the ground
	var ground_relative := velocity - ground_velocity
	if abs(ground_relative.dot(up_player)) > 0.01:
	return

It calculates the players velocity (relative to the ground) and attempts to ascertain if there is a significant vertical component to it indicating a jump or landing is in effect.

The problem occurs when the player is running up or down a slope which involves a vertical velocity with regards to the players "up" vector. Instead the test should be against the grounds normal vector - asking whether there is a vertical velocity moving towards or away from the ground surface.