refactor: vim.ui.clipboard #26040

Problem:
Platform-specific UI providers should live in `vim.ui.*`. #24164

Solution:
- Move `vim.clipboard.osc52` module to `vim.ui.clipboard.osc52`.
- TODO: move all of `clipboard.vim` to `vim.ui.clipboard`.

ref #25872

runtime/lua/vim/ui/clipboard/osc52.lua

@@ -0,0 +1,60 @@
+local M = {}
+
+function M.copy(lines)
+  local s = table.concat(lines, '\n')
+  io.stdout:write(string.format('\027]52;;%s\027\\', vim.base64.encode(s)))
+end
+
+function M.paste()
+  local contents = nil
+  local id = vim.api.nvim_create_autocmd('TermResponse', {
+    callback = function(args)
+      local resp = args.data ---@type string
+      local encoded = resp:match('\027%]52;%w?;([A-Za-z0-9+/=]*)')
+      if encoded then
+        contents = vim.base64.decode(encoded)
+        return true
+      end
+    end,
+  })
+
+  io.stdout:write('\027]52;;?\027\\')
+
+  local ok, res
+
+  -- Wait 1s first for terminals that respond quickly
+  ok, res = vim.wait(1000, function()
+    return contents ~= nil
+  end)
+
+  if res == -1 then
+    -- If no response was received after 1s, print a message and keep waiting
+    vim.api.nvim_echo(
+      { { 'Waiting for OSC 52 response from the terminal. Press Ctrl-C to interrupt...' } },
+      false,
+      {}
+    )
+    ok, res = vim.wait(9000, function()
+      return contents ~= nil
+    end)
+  end
+
+  if not ok then
+    vim.api.nvim_del_autocmd(id)
+    if res == -1 then
+      vim.notify(
+        'Timed out waiting for a clipboard response from the terminal',
+        vim.log.levels.WARN
+      )
+    elseif res == -2 then
+      -- Clear message area
+      vim.api.nvim_echo({ { '' } }, false, {})
+    end
+    return 0
+  end
+
+  -- If we get here, contents should be non-nil
+  return vim.split(assert(contents), '\n')
+end
+
+return M