StringBuilder: Add try_append_repeated() and append_repeated()

This two methods add the character as many times as specified by the
second parameter.

AK/StringBuilder.cpp

@@ -56,6 +56,14 @@ ErrorOr<void> StringBuilder::try_append(char ch)
     return {};
 }
 
+ErrorOr<void> StringBuilder::try_append_repeated(char ch, size_t n)
+{
+    TRY(will_append(n));
+    for (size_t i = 0; i < n; ++i)
+        TRY(try_append(ch));
+    return {};
+}
+
 void StringBuilder::append(StringView string)
 {
     MUST(try_append(string));
@@ -84,6 +92,11 @@ void StringBuilder::appendvf(char const* fmt, va_list ap)
         nullptr, fmt, ap);
 }
 
+void StringBuilder::append_repeated(char ch, size_t n)
+{
+    MUST(try_append_repeated(ch, n));
+}
+
 ByteBuffer StringBuilder::to_byte_buffer() const
 {
     // FIXME: Handle OOM failure.

AK/StringBuilder.h

@@ -35,6 +35,7 @@ public:
         return vformat(*this, fmtstr.view(), variadic_format_params);
     }
     ErrorOr<void> try_append(char const*, size_t);
+    ErrorOr<void> try_append_repeated(char, size_t);
     ErrorOr<void> try_append_escaped_for_json(StringView);
 
     void append(StringView);
@@ -46,6 +47,7 @@ public:
     void append_code_point(u32);
     void append(char const*, size_t);
     void appendvf(char const*, va_list);
+    void append_repeated(char, size_t);
 
     void append_as_lowercase(char);
     void append_escaped_for_json(StringView);