Add hexDigit combinator

## 🚀 Feature request

### Current Behavior

Currently there is no hexadecimal digit combinator available in `parser-ts/char`.

### Desired Behavior

The "core" library export a `hexDigit` combinator since it's quite common e.g. [Haskell](https://hackage.haskell.org/package/parsec-3.1.17.0/docs/src/Text.Parsec.Char.html#hexDigit) and [Purescript](https://github.com/purescript-contrib/purescript-parsing/blob/v10.0.0/src/Parsing/String/Basic.purs#L58-L58)

### Suggested Solution

```ts
const isHexDigit: (c: Char) => boolean = (c) =>
"0123456789abcdef".indexOf(c.toLowerCase()) !== -1;

const hexDigit: P.Parser = P.expected(P.sat(isHexDigit), "a hex digit");
```

### Who does this impact? Who is this for?

All users, regardless of skill would benefit from this. Not dependant on TS version.

### Describe alternatives you've considered

It's easy enough to build this yourself and put into your own codebase. I've done so while attempting to port over my own `uuid` and `mac address` parsers written in Purescript.

### Additional context

### Your environment

| Software | Version(s) |
| ---------- | ---------- |
| fp-ts | 2.16.1 |
| parser-ts | 0.7.0 |
| TypeScript | 5.2.2 |

Start in the parser-ts/char module, where character combinators are exported, and inspect nearby combinator tests if present. Compare the suggested isHexDigit and hexDigit behavior with existing character parsers. Done means the core library exports hexDigit, accepts 0-9 and a-f case-insensitively, rejects other characters, and has coverage for that behavior.

Written by the indexing model from the issue text.