Currently, `types.either` has support for the new valueMeta attribute
added by v2 check and merge, while `types.nullOr` does not.
The `lua` option deprecation warning implemented in
`modules/keymaps.nix` requires `valueMeta`, so re-implement `nullOr`
using `types.either` as a workaround.
The deprecation warning for the keymap-submocule `lua` option relied on
`getSubOptions`, however this is fundamentally flawed because that
function returns uses a different module eval from the one that merges
submodule definitions.
Since definitions are not used by `getSubOptions`,
`options.lua.isDefined` will never be true.
Instead, we have two choices:
1. Add a `luaIsDefined` option to the keymap submodule
2. Use the new v2 merge's `valueMeta` to access the actual module eval
