feat(io): add Output_Stream::append_padded_decimal_integer

I want to port some std::ostream-using code to Output_Stream. That
code uses set::setw, but Output_Stream has no equivalent. Implement
Output_Stream::append_padded_decimal_integer for unsigned integers so
we can port that code properly.

src/quick-lint-js/io/output-stream.h

@@ -9,6 +9,7 @@
 #include <quick-lint-js/io/file-handle.h>
 #include <quick-lint-js/io/file.h>
 #include <quick-lint-js/port/char8.h>
+#include <quick-lint-js/port/type-traits.h>
 #include <quick-lint-js/util/integer.h>
 #include <quick-lint-js/util/narrow-cast.h>
 
@@ -53,6 +54,37 @@ class Output_Stream {
     });
   }
 
+  // Write pad_character then write a decimal integer. The total number of
+  // characters written is at least pad_width.
+  //
+  // append_padded_decimal_integer(42, 4, '0')
+  // => u8"0042"
+  template <class T>
+  void append_padded_decimal_integer(
+      T value, int pad_width,
+      std::enable_if_t<std::is_unsigned_v<T>, Char8> pad_character) {
+    int max_width = std::max(integer_string_length<T>, pad_width);
+    this->append(max_width, [&](Char8* out) -> int {
+      Char8* integer_end = write_integer<T>(value, out);
+      int integer_width = narrow_cast<int>(integer_end - out);
+      bool need_padding = pad_width > integer_width;
+      if (!need_padding) {
+        return integer_width;
+      }
+
+      Char8* out_end = out + pad_width;
+      Char8* integer_start = std::copy_backward(out, integer_end, out_end);
+      QLJS_ASSERT(out < integer_start &&
+                  "should pad with at least one character");
+      QLJS_ASSERT(integer_start < out_end &&
+                  "integer should consume at least one character");
+      for (Char8* c = out; c != integer_start; ++c) {
+        *c = pad_character;
+      }
+      return pad_width;
+    });
+  }
+
   template <class T>
   void append_fixed_hexadecimal_integer(T value, int width) {
     this->append(width, [&](Char8* out) -> int {

test/test-output-stream.cpp

@@ -145,6 +145,55 @@ TEST(Test_Memory_Output_Stream, append_fixed_hexadecimal_integer) {
   }
 }
 
+TEST(Test_Memory_Output_Stream, append_padded_decimal_integer_exact_width) {
+  {
+    Memory_Output_Stream s(/*buffer_size=*/16);
+    s.append_padded_decimal_integer(1234u, 4, u8'x');
+    s.flush();
+    EXPECT_EQ(s.get_flushed_string8(), u8"1234"_sv);
+  }
+
+  {
+    Memory_Output_Stream s(/*buffer_size=*/16);
+    s.append_padded_decimal_integer(0u, 1, u8'x');
+    s.flush();
+    EXPECT_EQ(s.get_flushed_string8(), u8"0"_sv);
+  }
+}
+
+TEST(Test_Memory_Output_Stream, append_padded_decimal_integer_over_width) {
+  {
+    Memory_Output_Stream s(/*buffer_size=*/16);
+    s.append_padded_decimal_integer(1234u, 5, u8'x');
+    s.flush();
+    EXPECT_EQ(s.get_flushed_string8(), u8"x1234"_sv);
+  }
+
+  {
+    Memory_Output_Stream s(/*buffer_size=*/16);
+    s.append_padded_decimal_integer(0u, 20, u8' ');
+    s.flush();
+    EXPECT_EQ(s.get_flushed_string8(), u8"                   0"_sv);
+  }
+}
+
+TEST(Test_Memory_Output_Stream,
+     append_padded_decimal_integer_under_width_behaves_as_exact_width) {
+  {
+    Memory_Output_Stream s(/*buffer_size=*/16);
+    s.append_padded_decimal_integer(1234u, 1, u8'x');
+    s.flush();
+    EXPECT_EQ(s.get_flushed_string8(), u8"1234"_sv);
+  }
+
+  {
+    Memory_Output_Stream s(/*buffer_size=*/16);
+    s.append_padded_decimal_integer(0u, 0, u8' ');
+    s.flush();
+    EXPECT_EQ(s.get_flushed_string8(), u8"0"_sv);
+  }
+}
+
 #if defined(__EMSCRIPTEN__)
 // No filesystem on web.
 #else