fix: don't use unicodeSets regex feature, fix gradient transparency

The unicode sets / "v" flag are only supported since mid 2023; this
replaces them with [a-fA-F0-9], which matches the CSS spec.
https://caniuse.com/mdn-javascript_builtins_regexp_unicodesets

This also fixes support for 8-character #RRGGBBAA color codes.

Author: Aeledfyr

src/components/Markup.tsx

@@ -79,7 +79,7 @@ type CustomEntity = Entity & { type: "custom" };
 
 const TimeOffsetRegex = /^[+-]\d{4}$/;
 const CustomColorExprRegex =
-  /^(?<colors>#(?:\p{Hex_Digit}{3,4}|\p{Hex_Digit}{6,7})(?:-(?:#(?:\p{Hex_Digit}{3,4}|\p{Hex_Digit}{6,7})))+)\s+(?<text>.*)$/v;
+  /^(?<colors>#(?:[a-fA-F0-9]{3,4}|[a-fA-F0-9]{6,8})(?:-(?:#(?:[a-fA-F0-9]{3,4}|[a-fA-F0-9]{6,8})))+)\s+(?<text>.*)$/u;
 
 function transformCustomEntity(entity: CustomEntity, ctx: RenderContext) {
   const messages = useMessages();