AK: Rename Stream::format() to Stream::write_formatted()

This brings the function name in line with how we usually name those
functions, which is with a `read_` or `write_` prefix depending on what
they do.

While at it, make the internal `_impl` function private and not-virtual,
since there is no good reason to ever override that function.

AK/Stream.cpp

@@ -98,7 +98,7 @@ ErrorOr<void> Stream::write_until_depleted(ReadonlyBytes buffer)
     return {};
 }
 
-ErrorOr<void> Stream::format_impl(StringView fmtstr, TypeErasedFormatParams& parameters)
+ErrorOr<void> Stream::write_formatted_impl(StringView fmtstr, TypeErasedFormatParams& parameters)
 {
     StringBuilder builder;
     TRY(vformat(builder, fmtstr, parameters));