LibMarkdown: Convert render_to_terminal to String

This commit converts render_to_terminal from DeprecatedString to return
an ErrorOr<String>. This is to aid moving `man` away from
DeprecatedString.

I have opted not to convert render_to_html and render_to_inline_html for
now to keep this commit as small as possible.

Userland/Libraries/LibMarkdown/Document.cpp

@@ -43,15 +43,15 @@ DeprecatedString Document::render_to_inline_html() const
     return m_container->render_to_html();
 }
 
-DeprecatedString Document::render_for_terminal(size_t view_width) const
+ErrorOr<String> Document::render_for_terminal(size_t view_width) const
 {
     StringBuilder builder;
     for (auto& line : m_container->render_lines_for_terminal(view_width)) {
-        builder.append(line);
-        builder.append("\n"sv);
+        TRY(builder.try_append(line));
+        TRY(builder.try_append("\n"sv));
     }
 
-    return builder.to_deprecated_string();
+    return builder.to_string();
 }
 
 RecursionDecision Document::walk(Visitor& visitor) const

Userland/Libraries/LibMarkdown/Document.h

@@ -8,6 +8,7 @@
 
 #include <AK/DeprecatedString.h>
 #include <AK/OwnPtr.h>
+#include <AK/String.h>
 #include <LibMarkdown/Block.h>
 #include <LibMarkdown/ContainerBlock.h>
 
@@ -21,7 +22,7 @@ public:
     }
     DeprecatedString render_to_html(StringView extra_head_contents = ""sv) const;
     DeprecatedString render_to_inline_html() const;
-    DeprecatedString render_for_terminal(size_t view_width = 0) const;
+    ErrorOr<String> render_for_terminal(size_t view_width = 0) const;
 
     /*
      * Walk recursively through the document tree. Returning `RecursionDecision::Recurse` from

Userland/Utilities/man.cpp

@@ -7,6 +7,7 @@
 #include <AK/Assertions.h>
 #include <AK/ByteBuffer.h>
 #include <AK/DeprecatedString.h>
+#include <AK/String.h>
 #include <AK/Utf8View.h>
 #include <LibCore/ArgsParser.h>
 #include <LibCore/File.h>
@@ -100,7 +101,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     auto document = Markdown::Document::parse(source);
     VERIFY(document);
 
-    DeprecatedString rendered = document->render_for_terminal(view_width);
+    auto rendered = TRY(document->render_for_terminal(view_width));
     outln("{}", rendered);
 
     // FIXME: Remove this wait, it shouldn't be necessary but Shell does not

Userland/Utilities/md.cpp

@@ -56,7 +56,11 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         return 1;
     }
 
-    DeprecatedString res = html ? document->render_to_html() : document->render_for_terminal(view_width);
-    out("{}", res);
+    if (html) {
+        out("{}", document->render_to_html());
+    } else {
+        out("{}", TRY(document->render_for_terminal(view_width)));
+    }
+
     return 0;
 }