Everywhere: Replace single-char StringView op. arguments with chars

This prevents us from needing a sv suffix, and potentially reduces the
need to run generic code for a single character (as contains,
starts_with, ends_with etc. for a char will be just a length and
equality check).

No functional changes.

Userland/Utilities/stty.cpp

@@ -205,7 +205,7 @@ void print_human_readable(termios const& modes, winsize const& ws, bool verbose_
     auto escape_character = [&](u8 ch) {
         StringBuilder sb;
         if (ch <= 0x20) {
-            sb.append("^");
+            sb.append('^');
             sb.append(ch + 0x40);
         } else if (ch == 0x7f) {
             sb.append("^?"sv);