Fix Utf8Decoder operator== handling of the last code point in the input

[BillyONeal]

While working on diagnostics for #1210 I observed that we were printing the caret ^ in the wrong place when it goes after the input.

The way this works is we form the line of text to print, then decode the unicode encoding units, and when we hit the target, we stop and print ^:

vcpkg-tool/src/vcpkg/base/parse.cpp

Lines 51 to 68 in 5b8f9c4

	res.append_indent().append(msgFormattedParseMessageExpression, msg::value = line);
	res.append_raw('\n');
	auto caret_point = StringView{location.start_of_line.pointer_to_current(), location.it.pointer_to_current()};
	auto formatted_caret_point = msg::format(msgFormattedParseMessageExpression, msg::value = caret_point);
	std::string caret_string;
	caret_string.reserve(formatted_caret_point.data().size());
	for (char32_t ch : Unicode::Utf8Decoder(formatted_caret_point))
	{
	if (ch == '\t')
	caret_string.push_back('\t');
	else if (Unicode::is_double_width_code_point(ch))
	caret_string.append(" ");
	else
	caret_string.push_back(' ');
	}
	caret_string.push_back('^');

however, if the intended location for the ^ is the "end" of the line, we hit this bug:

vcpkg-tool/src/vcpkg/base/unicode.cpp

Line 273 in 5b8f9c4

return lhs.next_ == rhs.next_;

The iterator only compares the last_ pointers, but both the "points at the last code point in the input" and "points to the end of the input" state set next_ == last_. See:

vcpkg-tool/src/vcpkg/base/unicode.cpp

Lines 222 to 226 in 5b8f9c4

	if (next_ == last_)
	{
	current_ = end_of_file;
	return utf8_errc::NoError;
	}

This means that the points to the end and points one past the end iterator compare equal, so the loop in parse.cpp stops one position too early.

Also adds a bunch of testing for this specific case, for other parts of Utf8Decoder, adds a way to parse the first code point without failing, makes all the operators 'hidden friends', and removes localized strings for bugs-in-vcpkg-itself.

[Thomas1664]

noexcept could be added at more places.