Support CSS Color Module Level 4 (#12204)

doc/lua_api.txt

@@ -3248,7 +3248,7 @@ Colors
 `#RRGGBBAA` defines a color in hexadecimal format and alpha channel.
 
 Named colors are also supported and are equivalent to
-[CSS Color Module Level 3](https://www.w3.org/TR/css-color-3/).
+[CSS Color Module Level 4](https://www.w3.org/TR/css-color-4/#named-color).
 To specify the value of the alpha channel, append `#A` or `#AA` to the end of
 the color name (e.g. `colorname#08`).
 

src/util/string.cpp

@@ -494,6 +494,7 @@ const static std::unordered_map<std::string, u32> s_named_colors = {
 	{"plum",                 0xdda0dd},
 	{"powderblue",           0xb0e0e6},
 	{"purple",               0x800080},
+	{"rebeccapurple",        0x663399},
 	{"red",                  0xff0000},
 	{"rosybrown",            0xbc8f8f},
 	{"royalblue",            0x4169e1},