This is where "pure functions" can live, which can be shared by Nvim and
test logic which may not have a running Nvim instance available.
If in the future we use Nvim itself as the Lua engine for tests, then
these functions could be moved directly onto the `vim` Lua module.
closes#6580
ref #9280
Introduce the `vim.compat` module, to help environments with system Lua
5.2+ run the build/tests. Include the module implicitly in all tests.
ref #8677
legacy `vim` module:
beep
buffer
command
dict
eval
firstline
lastline
line
list
open
type
window