refactor: fix and improve utf8 utilities

[rept1d]

This PR solves the following problems:

• dpp::utf8substr is just broken: dpp::utf8substr("abcdefg", 2, 2) returns "cdef". Probably unnoticed due to very limited variety of usage scenarios.
• dpp::utf8substr and dpp::utf8len claim to have a specific result (empty string or zero, respectively) if the supplied string is not a valid UTF-8 sequence. In reality though, not only do they not verify if the string is valid UTF-8, they actually invoke UB for some inputs (e.g. dpp::utf8len("\xC0\xC0")) due to reading past the end of the string.
• The implementations of these functions are convoluted (use of goto, defining lots of variables in one line etc.)
• They only work with std::string's and not std::string_views. In particular, dp::utf8substr returns a new std::string, which causes allocations and copies that could be avoided in some cases.

What I did:

• Dropped the "guarantees" on what exactly these functions return in case of non-UTF-8 strings. We shouldn't pretend that they are provided. Now we claim that the result is "unspecified" (so no UB).
• Introduced a separate function called utf8subview which returns std::string_view instead of std::string, and made use of it in several places across the code base, where applicable.
• Replaced const std::string& with std::string_view in parameter declarations of these functions.
• Replaced std::string::size_type with size_t in parameter declarations of these functions because IMO it only makes everything unnecessarily verbose, especially if we assume that std::string::size_type might be different from std::string_view::size_type. It's almost never used in other places anyway, so why bother in this one in particular?
• Completely rewrote the implementations of both functions. One important difference in their behavior is that now they don't treat null terminators in the middle of strings as their end markers. These are now no different from any other ASCII character. It's also important that we don't rely on their presence at the end because std::string_views are not guaranteed to be null-terminated.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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