ci: add neovim 0.12 #165

Job	Run time
luals-check (0.12, luajit-master)	8m 20s
luals-check (nightly, luajit-master)	8m 19s
luals-check (0.12, 5.1)	8m 32s
luals-check (nightly, 5.1)	8m 48s
	33m 59s

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