backport: opts.scope in vim.ui.input (#39906)

feat(ui): vim.ui.input(opts.scope) #39570

Problem: There is no way for a `vim.ui.input` caller to indicate for
  which scope the input is. As in "This input is for something at cursor
  scope". This information can be useful for `vim.ui.input`
  implementation to tweak its behavior and presentation:
  - Show different floating window depending on the scope. For example:
    - Near cursor for "cursor" scope.
    - At line start for "line" scope.
    - In window corner for "buffer" and "window" scopes.
    - In whole editor corner for "tabpage", "editor", "project" scopes.
  - Navigate through history only for inputs with the same scope.

Solution: Document new `opts.scope` for `vim.ui.input`. Use it in the
  codebase.

runtime/lua/vim/lsp/buf.lua

@@ -815,6 +815,7 @@ function M.rename(new_name, opts)
 
         local prompt_opts = {
           prompt = 'New Name: ',
+          scope = 'cursor',
         }
         if result.placeholder then
           prompt_opts.default = result.placeholder
@@ -845,6 +846,7 @@ function M.rename(new_name, opts)
       local prompt_opts = {
         prompt = 'New Name: ',
         default = cword,
+        scope = 'cursor',
       }
       vim.ui.input(prompt_opts, function(input)
         if not input or #input == 0 then