feat(diff): improve diff application and buffer handling

Refactored diff handling in mappings to use unified diff parsing and
application. Removed legacy line-numbered diff logic and replaced with
buffer-aware unified diff utilities. Updated prompts and resource block
generation to match new diff format. Improves reliability and accuracy
when applying, showing, and yanking diffs in chat.

Signed-off-by: Tomas Slusny <[email protected]>

Author: deathbeam

lua/CopilotChat/config.lua

@@ -19,6 +19,7 @@
 ---@field tools string|table<string>|nil
 ---@field resources string|table<string>|nil
 ---@field sticky string|table<string>|nil
+---@field diff 'block'|'unified'?
 ---@field language string?
 ---@field temperature number?
 ---@field headless boolean?
@@ -62,6 +63,7 @@ return {
   tools = nil, -- Default tool or array of tools (or groups) to share with LLM (can be specified manually in prompt via @).
   resources = 'selection', -- Default resources to share with LLM (can be specified manually in prompt via #).
   sticky = nil, -- Default sticky prompt or array of sticky prompts to use at start of every new chat (can be specified manually in prompt via >).
+  diff = 'block', -- Default diff format to use, 'block' or 'unified'.
   language = 'English', -- Default language to use for answers
 
   temperature = 0.1, -- Result temperature