API to get prompt's *starting* Y coordinate, for external tab-completers that must scroll terminal and thus relocate prompt position

Start by tracing the existing InvokePrompt entry point and PSReadLine's prompt and cursor-position handling. Compare the proposed GetPromptStartPosition and MovePromptYPosition APIs, then define behavior for multiline prompts and terminal scrolling; done means an external completer can restore the prompt using its starting Y coordinate.

Prerequisites

• Write a descriptive title.

Description of the new feature/enhancement

PSFzf exposes an alternative to MenuComplete using fzf's fuzzy matching menu. Just like MenuComplete, fzf's menu requires X lines of space beneath the prompt, so it may scroll the terminal upward when activated. When it returns, PSReadLine needs to be informed that the prompt has moved upward to a different Y position.

When the prompt is multiple lines long, this Y position is not the same as (Get-Host).UI.RawUI.CursorPosition. RawUI shows us the end of the prompt, but not the beginning. The beginning is the value which must be passed to InvokePrompt.

Related issues:
https://github.com/kelleyma49/PSFzf/issues/202#issuecomment-2503800370
https://github.com/kelleyma49/PSFzf/issues/227

Proposed technical implementation details (optional)

There are hacks to compute the prompt height, by overriding function prompt to capture the Y coordinate before the prompt is rendered. But the correct solution is for PSReadLine to expose either:

• GetPromptStartPosition to get the Y coordinate of the prompt, so fzf can subtract the number of lines it scrolled the terminal and pass the result to InvokePrompt
• MovePromptYPosition if you really don't want to expose the above. This would accept a positive or negative number and add it to the prompt's Y position.

Pseudo-code to show how this might work:

Set-PSReadLineKeyHandler -Key Tab -ScriptBlock {
  $cursor_y = (Get-Host).UI.RawUI.CursorPosition.Y # end of prompt, to account for multi-line prompts
  $prompt_y = [PSReadline]::GetPromptStartPosition().Y # start of prompt
  InvokeFzfMenu # This may scroll the terminal to make space for the menu, and will thus restore cursor position to a new (higher) Y coordinate when finished
  # How much did cursor position move?  Add this delta to prompt's start position
  $new_prompt_y = $prompt_y + (Get-Host).UI.RawUI.CursorPosition.Y - $cursor_y
  [PSReadline]::InvokePrompt($null, $new_prompt_y)
}