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.

Kernel/CommandLine.cpp

@@ -58,7 +58,7 @@ UNMAP_AFTER_INIT NonnullOwnPtr<KString> CommandLine::build_commandline(StringVie
     StringBuilder builder;
     builder.append(cmdline_from_bootloader);
     if constexpr (!s_embedded_cmd_line.is_empty()) {
-        builder.append(" ");
+        builder.append(' ');
         builder.append(s_embedded_cmd_line);
     }
     return KString::must_create(builder.string_view());