LibLine: Avoid refreshing the entire line when inserting at the end

This patchset allows the editor to avoid redrawing the entire line when
the changes cause no unrecoverable style updates, and are at the end of
the line (this applies to most normal typing situations).
Cases that this does not resolve:
- When the cursor is not at the end of the buffer
- When a display refresh changes the styles on the already-drawn parts
  of the line
- When the prompt has not yet been drawn, or has somehow changed

Fixes #5296.

Meta/CMake/all_the_debug_macros.cmake

@@ -168,6 +168,7 @@ set(DEBUG_AUTOCOMPLETE ON)
 set(FILE_WATCHER_DEBUG ON)
 set(SYSCALL_1_DEBUG ON)
 set(RSA_PARSE_DEBUG ON)
+set(LINE_EDITOR_DEBUG ON)
 
 # False positive: DEBUG is a flag but it works differently.
 # set(DEBUG ON)