This adds the `nixvimExtend` attribute to the generated standalone
derivation, this attribute takes a module as an argument and returns a
new standalone derivation with the initial module & the extended module
merged together.
In our basic template we used to provide a check based on
`mkTestDerivationFromNvim`. The issue with this check (that is handled
correctly internally) is that some plugins _can't_ be used in the test
environment, for example image.nvim like in #1085.
This commit introduces a new function to generate such checks,
`mkTestDerivationFromNixvimModule`, that wraps a nixvim configuration
instead of a built nvim instance.
Then a configuration can rely on the newly added
`helpers.enableExceptInTests` attribute to disable parts of the
configuration depending if it is evaluated in tests or in a real final
configuration.
Resolves#1085
