Define a CMake target for every subdirectory of test/functional that
contains functional tests, and a functionaltest_parallel target that
depends on all those targets, allowing multiple test runners to run in
parallel.
On CI, use at most 2 parallel test runners, as using more may increase
system load and make tests unstable.
Problem:
- Exposing the raw config as table is a pattern not seen anywhere else
in the Nvim codebase.
- Old spellfile.vim docs still available, no new documentation
Solution:
- Exposing a `config()` function that both acts as "getter" and "setter"
is a much more common idiom (e.g. vim.lsp, vim.diagnostic).
- Add new documentation and link old docs to |spellfile.lua| instead of
|spellfile.vim|.
