AK: Add an exact and fast hex float parsing algorithm

Similar to decimal floating point parsing the current strtod hex float
parsing gives a lot of incorrect results. We can use a similar technique
as with decimal parsing however hex floats are much simpler as we don't
need to scale with a power of 5.

For hex floats we just provide the parse_first_hexfloat API as there is
currently no need for a parse_hexfloat_completely API.

Again the accepted input for parse_first_hexfloat is very lenient and
any validation should be done before calling this method.

AK/FloatingPointStringConversions.h

@@ -62,7 +62,24 @@ FloatingPointParseResults<T> parse_first_floating_point_until_zero_character(cha
 template<FloatingPoint T = double>
 Optional<T> parse_floating_point_completely(char const* start, char const* end);
 
+/// This function finds the first floating point as a hex float within [start, end).
+/// The accepted format is intentionally as lenient as possible. If your format is
+/// stricter you must validate it first. The format accepts:
+/// - An optional sign, both + and - are supported
+/// - Optionally either 0x or OX
+/// - 0 or more hexadecimal digits, with leading zeros allowed [1]
+/// - A decimal point '.', which can have no digits after it
+/// - 0 or more hexadecimal digits, unless the first digits [1] doesn't have any digits,
+///   then this must have at least one
+/// - An exponent 'p' or 'P' followed by an optional sign '+' or '-' and at least one decimal digit
+/// NOTE: The exponent is _not_ hexadecimal and gives powers of 2 not 16.
+/// This function additionally detects out of range values which have been rounded to
+/// [-]infinity or 0 and gives the next character to read after the floating point.
+template<FloatingPoint T = double>
+FloatingPointParseResults<T> parse_first_hexfloat_until_zero_character(char const* start);
+
 }
 
 using AK::parse_first_floating_point;
+using AK::parse_first_hexfloat_until_zero_character;
 using AK::parse_floating_point_completely;