[Issue 23376] Allow multi-code-point HTML entities

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23376

--- Comment #4 from Dennis <dkorpel at live.nl> ---
(In reply to Dennis from comment #3)
> Looking at the source code, it seems like the only reason is because no one
> bothered to implement it. `Lexer.escapeSequence` returns a single `dchar`,
> so it would require a bit of refactoring.

Oh, there's also the fact that escape sequences are also used in character
literals, so you need to account for this:

```
dchar x = "&acE"; // Requires two dchars, 0x0223E;0x00333
```

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