Implement unicode character replacements

Useful to draw whitespaces with alternate characters and colors
without slowing down the text rendering.

A new API is implemented. A renderer.replacements object can be created
to list the replacements.

In turns the function renderer.draw_text and draw_text_subpixel now accept
two optional arguments for replacements.

changelog.md

@@ -6,6 +6,12 @@ This files document the changes done in Lite XL for each release.
 
 Add `system.set_window_opacity` function.
 
+Add codepoint replacement API to support natively the "draw whitespaces" option.
+It replace the `drawwhitespace` plugin. If can be configured using the
+`config.draw_whitespace` boolean variable and enabled and disables using the
+commands `draw-whitespace:toggle`, `draw-whitespace:enable`,
+`draw-whitespace:disable`.
+
 ### 1.16.5
 
 Hotfix for Github's issue https://github.com/franko/lite-xl/issues/122