AK/Hex: Decode hex digit in constexpr context

Problem:
- Hex digit decoding is not `constexpr`, but can be.

Solution:
- Move the body of the function to the header and decorate with
  `constexpr`.
- Provide tests for run-time and compile-time evaluation.

AK/Hex.cpp

@@ -35,17 +35,6 @@
 
 namespace AK {
 
-u8 decode_hex_digit(char digit)
-{
-    if (digit >= '0' && digit <= '9')
-        return digit - '0';
-    if (digit >= 'a' && digit <= 'f')
-        return 10 + (digit - 'a');
-    if (digit >= 'A' && digit <= 'F')
-        return 10 + (digit - 'A');
-    return 255;
-}
-
 Optional<ByteBuffer> decode_hex(const StringView& input)
 {
     if ((input.length() % 2) != 0)