Use floating window for input prompt.

[hsanson]

• Replace the input prompt to use a floating window relative to the cursor instead of vim.ui.input. This makes sending prompts to opencode more pleasant visually and improves usability.
  • To achieve this, many of the xxx_persistent_prompt() methods were renamed and refactored to support a type argument that determines if the prompt window is shown as a floating window or a persistent window at the bottom.
• Refactor the configuration to a separate module that would allow to extend it easily to support other features.
  • Added some configurations to allow users to set height and width of the floating and persistent prompts.
• When opening input prompt prefill it with "@cursor" by default or "@selection" if something is visually selected.
• Added some utilty methods:
  • safe_size, safe_height, and safe_width used to ensure height and width set in configuration have sane values.
    • If given a float between 0 and 1 as input it computes the height/width as a percentage of the window height/width.
  • add_text_at_cursor() method is used to prefill the prompt buffer with text (e.g. @cursor/@selection). It support filling the text from the cursor position and updates the cursor to be at the end of the inserted text. Supports single and multi line text. (Note: generated using Claude 4.0 LLM model).
  • Bonus: allow configuring the icon shown in the prompt title.