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/mount.cpp

@@ -70,7 +70,7 @@ static int get_source_fd(StringView source)
 static bool mount_by_line(String const& line)
 {
     // Skip comments and blank lines.
-    if (line.is_empty() || line.starts_with("#"))
+    if (line.is_empty() || line.starts_with('#'))
         return true;
 
     Vector<String> parts = line.split('\t');