Implement styled arguments (#2793)

* Implement styled arguments

* Inherit from formatter<Arg> to get the underlying `parse` and `format`

* Move styled_arg definition into the previous detail block

* Change styled_arg ctor parameters names to avoid shadowing members

* Move const before auto

* Remove redundant constructor for styled_arg

* Use the iterator instead of the buffer in styled_arg::format

* Remove unnecessary `styled` overloads

* Remove defaulted text_style parameter in styled function

test/color-test.cc

@@ -50,6 +50,12 @@ TEST(color_test, format) {
             "\x1b[105mtbmagenta\x1b[0m");
   EXPECT_EQ(fmt::format(fg(fmt::terminal_color::red), "{}", "foo"),
             "\x1b[31mfoo\x1b[0m");
+  EXPECT_EQ(fmt::format("{}{}", fmt::styled("red", fg(fmt::color::red)),
+                        fmt::styled("bold", fmt::emphasis::bold)),
+            "\x1b[38;2;255;000;000mred\x1b[0m\x1b[1mbold\x1b[0m");
+  EXPECT_EQ(fmt::format("{}", fmt::styled("bar", fg(fmt::color::blue) |
+                                                     fmt::emphasis::underline)),
+            "\x1b[4m\x1b[38;2;000;000;255mbar\x1b[0m");
 }
 
 TEST(color_test, format_to) {