fix(vim.ui)!: change open() to return result|nil, errmsg|nil #28612

reverts e0d92b9cc2 #28502

Problem:
`vim.ui.open()` has a `pcall()` like signature, under the assumption
that this is the Lua idiom for returning result-or-error. However, the
`result|nil, errmsg|nil` pattern:
- has precedent in:
  - `io.open`
  - `vim.uv` (`:help luv-error-handling`)
- has these advantages:
  - Can be used with `assert()`:
    ```
    local result, err = assert(foobar())
    ```
  - Allows LuaLS to infer the type of `result`:
    ```
    local result, err = foobar()
    if err then
      ...
    elseif result then
      ...
    end
    ```

Solution:
- Revert to the `result|nil, errmsg|nil` pattern.
- Document the pattern in our guidelines.

runtime/doc/develop.txt

@@ -309,6 +309,11 @@ See also |dev-naming|.
   - return iterable instead of table
   - mimic the pairs() or ipairs() interface if the function is intended to be
     used in a "for" loop.
+  - when a result-or-error interface is needed, return `result|nil, errmsg|nil`: >
+    ---@return Foo|nil # Result object, or nil if not found.
+    ---@return nil|string # Error message on failure, or nil on success.
+<
+    - Examples: |vim.ui.open()| |io.open()| |luv-error-handling|
 
 Interface conventions ~