Implement vgoto

[bi4k8]

Checklist

• I have described the changes
• I have linked to any relevant GitHub issues, if applicable
• Documentation in doc/ has been updated
• All new code is licensed under GPLv3

Description

This implements a vertical goto, to match the existing horizontal one. This has been requested by a number of users. Closes #450, and is based in part on the patch there by @easysid.

Tested against the following conkyrc:

conky.config = {
	out_to_wayland = true,
	use_xft= true,
	font= 'DejaVu Sans:style=medium:size=9'
}
conky.text = [[
foo bar${vgoto 0}${goto 0}text after vgoto

line 1
line 2
another
line 4
line 5
]]

This is still a draft because it seems to cause some issues calculating height--with the above conkyrc, on the first iteration, everything looks as expected, but at the beginning of the second iteration, the whole displayed output moves downward, and the bottom line gets cut off. I'll have to debug what's happening there. I have seen similar jumps previously, so it seems more likely that this is exacerbating an existing misbehavior than introducing a completely new one.

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[Caellian]

Just letting you know that node kinds (variables) now use an enum class, so instead of VGOTO you'll have to write text_node_t::VGOTO. That seems to be the only merge issue introduced by my recent PR.