refactor!: remove nvim and provider module for checkhealth

The namespacing for healthchecks for neovim modules is inconsistent and
confusing. The completion for `:checkhealth` with `--clean` gives

```
nvim
provider.clipboard
provider.node
provider.perl
provider.python
provider.ruby
vim.lsp
vim.treesitter
```

There are now three top-level module names for nvim: `nvim`, `provider`
and `vim` with no signs of stopping. The `nvim` name is especially
confusing as it does not contain all neovim checkhealths, which makes it
almost a decoy healthcheck.

The confusion only worsens if you add plugins to the mix:

```
lazy
mason
nvim
nvim-treesitter
provider.clipboard
provider.node
provider.perl
provider.python
provider.ruby
telescope
vim.lsp
vim.treesitter
```

Another problem with the current approach is that it's not easy to run
nvim-only healthchecks since they don't share the same namespace. The
current approach would be to run `:che nvim vim.* provider.*` and would
also require the user to know these are the neovim modules.

Instead, use this alternative structure:

```
vim.health
vim.lsp
vim.provider.clipboard
vim.provider.node
vim.provider.perl
vim.provider.python
vim.provider.ruby
vim.treesitter
```

and

```
lazy
mason
nvim-treesitter
telescope
vim.health
vim.lsp
vim.provider.clipboard
vim.provider.node
vim.provider.perl
vim.provider.python
vim.provider.ruby
vim.treesitter
```

Now, the entries are properly sorted and running nvim-only healthchecks
requires running only `:che vim.*`.

runtime/lua/vim/provider/ruby/health.lua

@@ -0,0 +1,69 @@
+local health = vim.health
+local iswin = vim.loop.os_uname().sysname == 'Windows_NT'
+
+local M = {}
+
+function M.check()
+  health.start('Ruby provider (optional)')
+
+  if health.provider_disabled('ruby') then
+    return
+  end
+
+  if vim.fn.executable('ruby') == 0 or vim.fn.executable('gem') == 0 then
+    health.warn(
+      '`ruby` and `gem` must be in $PATH.',
+      'Install Ruby and verify that `ruby` and `gem` commands work.'
+    )
+    return
+  end
+  health.info('Ruby: ' .. health.system({ 'ruby', '-v' }))
+
+  local host, _ = vim.provider.ruby.detect()
+  if (not host) or host:find('^%s*$') then
+    health.warn('`neovim-ruby-host` not found.', {
+      'Run `gem install neovim` to ensure the neovim RubyGem is installed.',
+      'Run `gem environment` to ensure the gem bin directory is in $PATH.',
+      'If you are using rvm/rbenv/chruby, try "rehashing".',
+      'See :help g:ruby_host_prog for non-standard gem installations.',
+      'You may disable this provider (and warning) by adding `let g:loaded_ruby_provider = 0` to your init.vim',
+    })
+    return
+  end
+  health.info('Host: ' .. host)
+
+  local latest_gem_cmd = (iswin and 'cmd /c gem list -ra "^^neovim$"' or 'gem list -ra ^neovim$')
+  local ok, latest_gem = health.cmd_ok(vim.split(latest_gem_cmd, ' '))
+  if not ok or latest_gem:find('^%s*$') then
+    health.error(
+      'Failed to run: ' .. latest_gem_cmd,
+      { "Make sure you're connected to the internet.", 'Are you behind a firewall or proxy?' }
+    )
+    return
+  end
+  local gem_split = vim.split(latest_gem, [[neovim (\|, \|)$]])
+  latest_gem = gem_split[1] or 'not found'
+
+  local current_gem_cmd = { host, '--version' }
+  local current_gem
+  ok, current_gem = health.cmd_ok(current_gem_cmd)
+  if not ok then
+    health.error(
+      'Failed to run: ' .. table.concat(current_gem_cmd, ' '),
+      { 'Report this issue with the output of: ', table.concat(current_gem_cmd, ' ') }
+    )
+    return
+  end
+
+  if vim.version.lt(current_gem, latest_gem) then
+    local message = 'Gem "neovim" is out-of-date. Installed: '
+      .. current_gem
+      .. ', latest: '
+      .. latest_gem
+    health.warn(message, 'Run in shell: gem update neovim')
+  else
+    health.ok('Latest "neovim" gem is installed: ' .. current_gem)
+  end
+end
+
+return M