Certain values of 'wildignore', .e.g `*/node_modules/**`, would make the
provider checks not find the right executables.

Fixes https://github.com/neovim/neovim/issues/14388

Problem: ruby#Detect() and node#Detect() don't return a [prog, err] pair
which means callers must special-case them.

Solution: align their return signatures with the perl/pythonx providers.

* tv_to_argv: error when cmd is not executable
  Callers always assume that emsg was emitted:
  - https://github.com/neovim/neovim/blob/57fbf288/src/nvim/eval.c#L12509
  - https://github.com/neovim/neovim/blob/57fbf288/src/nvim/eval.c#L17923
  - https://github.com/neovim/neovim/blob/57fbf288/src/nvim/eval.c#L18202
* test/functional/provider: display reason from missing_provider
* provider#node#Detect: skip / handle non-existing node executable

Value of 1 cannot be used, because users might set that in their vimrc
to _disable_ a provider, which would confuse :checkhealth and has().

Instead of deciding provider status in eval_has_provider, move the
decision to the provider Vim scripts.

Previously, provider loading worked as follows:

1. eval_has_provider() verified provider availability by searching for
   the provider#providername#Call function and cached this verificaion as a static
   variable for some providers
2. providers short-circuited on loading to prevent the definition of the
   Call function (with the exception of the node provider that did not)

This commit changes the expected interface between nvim and its
providers to facilitate provider reloading, by splitting the
verification of the provider from the availability of the Call function.

eval_has_provider() now checks for a provider#providername#enabled
variable. It is up to the provider script to set this to 0 or 1
accordingly. eval_call_provider() remains unchanged.

All providers hosting a Call function were updated to respect this.

The clipboard provider now has a Reload function to reload the
provider.

Before this commit, if user does this:
    let g:node_host_prog = '~/.nvm/versions/node/v11.3.0/bin/neovim-node-host'
the "~/" is not expanded to user's home directory.

`:help g:ruby_host_prog` suggests a path with "~/" so technically we
already claimed to support this.

closes https://github.com/neovim/node-client/issues/102

ref https://github.com/neovim/neovim/issues/9001#issuecomment-434802234

For each package manager,
1. Check if they are executable before starting a job.
2. Check if the job opts are set before checking the job status
   TODO: Validate the job id.

Prioritize npm because it's faster than yarn.
Reindent the code to 2-space indent.