refactor: rewrite clipboard provider healthchecks in Lua

This is required to remove the vimscript checkhealth functions.

runtime/lua/provider/health.lua

@@ -0,0 +1,55 @@
+local M = {}
+
+-- Returns true if `cmd` exits with success, else false.
+local function cmd_ok(cmd)
+  vim.fn.system(cmd)
+  return vim.v.shell_error == 0
+end
+
+local function executable(exe)
+  return vim.fn.executable(exe) == 1
+end
+
+local function is_blank(s)
+  return s:find('^%s*$') ~= nil
+end
+
+local function clipboard()
+  vim.health.report_start('Clipboard (optional)')
+
+  if
+    os.getenv('TMUX')
+    and executable('tmux')
+    and executable('pbpaste')
+    and not cmd_ok('pbpaste')
+  then
+    local tmux_version = string.match(vim.fn.system('tmux -V'), '%d+%.%d+')
+    local advice = {
+      'Install tmux 2.6+.  https://superuser.com/q/231130',
+      'or use tmux with reattach-to-user-namespace.  https://superuser.com/a/413233',
+    }
+    vim.health.report_error('pbcopy does not work with tmux version: ' .. tmux_version, advice)
+  end
+
+  local clipboard_tool = vim.fn['provider#clipboard#Executable']()
+  if vim.g.clipboard and is_blank(clipboard_tool) then
+    local error_message = vim.fn['provider#clipboard#Error']()
+    vim.health.report_error(
+      error_message,
+      "Use the example in :help g:clipboard as a template, or don't set g:clipboard at all."
+    )
+  elseif is_blank(clipboard_tool) then
+    vim.health.report_warn(
+      'No clipboard tool found. Clipboard registers (`"+` and `"*`) will not work.',
+      ':help clipboard'
+    )
+  else
+    vim.health.report_ok('Clipboard tool found: ' .. clipboard_tool)
+  end
+end
+
+function M.check()
+  clipboard()
+end
+
+return M