selected-window-accent-mode with window colour blending options

I have enhanced selected-window-accent functionality with blending options to blend colourize the selected window (see below):

This means that not only will the mode-line, fringes and header be accent coloured, but also the whole selected window background colour, tinted to the alpha chosen.

The features added are as follows:

Introduced selected-window-accent-use-blend-background and selected-window-accent-use-blend-alpha to allow blending of the accent colour with the background of the selected window a selected alpha amount.
Introduced new toggle functions and added to transient:
- selected-window-accent-toggle-blend-background
- selected-window-accent-toggle-pywal
Added function selected-window-accent-blend-colors to support blending of two colours.
Updated selected-window-accent--set-foreground-color and selected-window-accent functions to incorporate new blending feature.
Improved existing functions for setting and toggling accent features, ensuring consistent style and formatting.
Cleaned up indentation and formatting inconsistencies across the file for better readability.

Example use of new selected window blending functionality:

(use-package selected-window-accent-mode
  :config (selected-window-accent-mode 1)
  :custom
  (selected-window-accent-fringe-thickness 10)
  (selected-window-accent-use-blend-background t)
  (selected-window-accent-use-blend-alpha 0.2)
  (selected-window-accent-tab-accent t)
  (selected-window-accent-custom-color "cyan4")
  (selected-window-accent-mode-style 'default))

(use-package selected-window-accent-mode
  :config (selected-window-accent-mode 1)
  :custom
  (selected-window-accent-fringe-thickness 10)
  (selected-window-accent-use-blend-background t)
  (selected-window-accent-use-blend-alpha 0.1)
  (selected-window-accent-tab-accent t)
  (selected-window-accent-custom-color "orange")
  (selected-window-accent-mode-style 'subtle))