feat: add option to disable modal dim effect

For some terminal themes is cooler to not dim the screen when palettes
are showing.

src/config.zig

@@ -9,6 +9,7 @@ gutter_line_numbers_style: DigitStyle = .ascii,
 gutter_symbols: bool = true,
 enable_terminal_cursor: bool = true,
 enable_terminal_color_scheme: bool = false,
+enable_modal_dim: bool = true,
 highlight_current_line: bool = true,
 highlight_current_line_gutter: bool = true,
 highlight_columns: []const u16 = &.{ 80, 100, 120 },