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/Shell/SyntaxHighlighter.cpp

@@ -128,7 +128,7 @@ private:
             span.attributes.color = m_palette.syntax_keyword();
             span.attributes.bold = true;
             m_is_first_in_command = false;
-        } else if (node->text().starts_with("-")) {
+        } else if (node->text().starts_with('-')) {
             span.attributes.color = m_palette.syntax_preprocessor_statement();
         } else {
             span.attributes.color = m_palette.base_text();