Replace `utf8Parser` with `til::u8u16` in order to have the same
conversion algorithms used in terminal and conhost.
This PR addresses item 2 in this list:
1. ✉ Implement `til::u8u16` and `til::u16u8` (done in PR #4093)
2. ✔ **Unify UTF-8 handling using `til::u8u16` (this PR)**
2.1. ✔ **Update VtInputThread::_HandleRunInput()**
2.2. ✔ **Update ApiRoutines::WriteConsoleAImpl()**
2.3. ❌ (optional / ask the core team) Remove Utf8ToWideCharParser from the code base to avoid further use
3. ❌ Enable BOM discarding (follow up)
3.1. ❌ extend `til::u8u16` and `til::u16u8` with a 3rd parameter to enable discarding the BOM
3.2. ❌ Make use of the 3rd parameter to discard the BOM in all current function callers, or (optional / ask the core team) make it the default for `til::u8u16` and `til::u16u8`
4. ❌ Find UTF-16 to UTF-8 conversions and examine if they can be unified, too (follow up)
Closes#4086Closes#3378
