Fix overrideable hand ToolCapabilities and range

[cx384]

Fixes #15740

To avoid confusion, notice:
selected_item is the item selected in the hotbar, ("" if empty)
hand_item is the overwritten hand ("" by default).
tool_item/playeritem/punchitem is what should be used for getToolCapabilities (getWieldedItem returns this)

getToolCapabilities needs both tool_item and hand_item as a fallback.

Callbacks and node placement still only uses selected_item.

To do

Ready for Review.

How to test

• Like explained in the issue, try e.g.

core.register_item("default:hand", {
	type = "none",
	range = 20,
	tool_capabilities = {
		groupcaps = {
			crumbly = {times={[1] = 0.1, [2] = 0.1, [3] = 0.1}, uses = 0, maxlevel = 1},
			cracky = {times={[1] = 0.1, [2] = 0.1, [3] = 0.1}, uses = 0, maxlevel = 1},
		}
	}
})

core.register_chatcommand('hand', {
	func = function(name, param)
		local player = core.get_player_by_name(name)

		local handitem
		if param == 'reset' then
			handitem = ''
		else
			handitem = 'default:hand'
		end

		local inv = player:get_inventory()
		inv:set_size('hand', 1)
		inv:set_stack('hand', 1, ItemStack(handitem))
	end
})

• See that it works while wielding nothing, and while e.g. a node

[appgurueu]

Mostly nitpicks. Works as expected in testing.

The code looks sensible: It adds the hand as a parameter to getToolCapabilities, and then the rest are primarily changes to pass the hand around.

[cx384]

Thanks for reviewing, applied everything.
(At first I added the explicit nullptr comparisons, since I thought we may want them for readability considering the old code, but they are gone now.)

About the changes this PR makes, there is also a replacement of selected_item with tool_item.
This is needed, since selected_item is never the hand, it can only be empty (which is equal to the default hand ""), and this would then be preferred by getToolCapabilities instead of the actual hand.
And as I already wrote, the item callbacks still only use selected_item.
(Changing this may cause some problems in case a mod allows using an arbitrary item as hand.)