feat(ui): add backdrop (#1759)

Adds a backdrop for the Mason window. Can be disabled by setting the `ui.backdrop` option:

```lua
require("mason").setup {
  ui = {
    backdrop = 100
  }
}
```

The backdrop is not displayed if `'termguicolors'` is not enabled or if Neovim is transparent.

Co-authored-by: William Boman <william@redwill.se>

1 files changed, 4 insertions, 0 deletions

diff --git a/README.md b/README.md
index 9e33b653..b64d671d 100644
--- a/README.md
+++ b/README.md
@@ -269,6 +269,10 @@ local DEFAULT_SETTINGS = {
         -- The border to use for the UI window. Accepts same border values as |nvim_open_win()|.
         border = "none",
 
+        ---@since 1.11.0
+        -- The backdrop opacity. 0 is fully opaque, 100 is fully transparent.
+        backdrop = 60,
+
         ---@since 1.0.0
         -- Width of the window. Accepts:
         -- - Integer greater than 1 for fixed width.